With control of the Senate at stake, Republicans and Democrats are fiercely battling in at least nine states with competitive races.
Republicans entered the 2016 election cycle on the defensive: Of the 34 seats up for grabs this year, 24 are currently held by the GOP and just ten by the Democrats. Eight of the nine most competitive races are for GOP-held seats and six of those contests feature first-term GOP incumbents who were elected in the tea party wave of 2010.
The GOP currently holds the majority with 54 seats, meaning that Democrats need a net gain of four seats to win control of the upper chamber for the next two years if Hillary Clinton wins the presidency or a net gain of five should Donald Trump prevail.
Voters are indicating that they want a congressional check on the president. While Clinton led by 4 points in the final pre-election NBC News/Wall Street Journal poll, 51 percent of likely voters said they were more likely to support a Republican congressional candidate who would serve as a check on a potential Clinton presidency. Just 40 percent said they would be more likely to support a Democratic congressional candidate who would support Clinton's agenda. (Still, when voters were asked if they prefer a Congress led by Democrats or by Republicans, Clinton's party retained an advantage, 47 percent to 44 percent.)
Last-minute cash infusions by the parties and by super PACs are keeping Senate races competitive and outcomes uncertain.
With Election Day just two days away, here are nine races that could determine U.S. Senate control:
1). Illinois: Republican Sen. Mark Kirk v Democrat Tammy Duckworth
Kirk was elected to the Senate to fill the remaining days of Sen. Barack Obama's seat in 2010 while simultaneously winning his own six-year term. Kirk could very well be a one-term Senator, however, as polls in this race show Kirk is seven points behind challenger Duckworth.
Kirk has taken some moderate positions to better represent his Democratic-leaning state, including his support of gun control legislation and a vote on Obama's Supreme Court nominee Merrick Garland. He has also separated himself from Trump, being one of the first to say he would not support the nominee of his party, even running anti-Trump television ads, including this Spanish-language one.
But Kirk's controversial remarks have eclipsed his positions. He recently called President Barack Obama a "drug dealer in chief," but the most egregious comment was less than two weeks before Election Day when he said during a debate that Duckworth's parents came to the U.S. to "serve" George Washington. Kirk apologized for racially offensive remarks
While the race is more a referendum on Kirk, Duckworth, his challenger, is running on her record. As an Iraq War vet who lost both her legs while piloting a Black Hawk helicopter, she spent much of her post-war career on veterans issues, eventually winning election to the House in 2012.
Duckworth was endorsed by the right-leaning Chicago Tribune editorial board, citing Kirk's health as a reason. Kirk suffered a stroke in 2012 and in an attempt to dispel any concerns from voters, he released a letter from his doctor recently that said he has made "a full cognitive recovery."
2). Wisconsin: Republican Sen. Ron Johnson v Democratic former Sen. Russ Feingold
This is a re-match from 2010 when upstart Johnson beat 18-year incumbent Feingold in the tea party wave of 2010. Six years later, the match-up between the conservative Johnson and the liberal Feingold, is much closer than anticipated.
Early in the race, Feingold dominated the polls, resulting in the national super PACs and the respective campaign committees to stop spending money there. But the race has become nearly tied in the final weeks.
Attempting to walk a very fine line, Johnson has not defended Trump but hasn't denounced him either; he supports him but won't endorse him. At a debate earlier this month Johnson said he agreed with the nominee on issues like border security but that he won't "defend the indefensible."
Feingold is attempting to paint Johnson as a rigid conservative who is too extreme for Wisconsin while Johnson says that Feingold has spent too much time in Washington and will say anything to get elected.
3). Pennsylvania: Republican Sen. Pat Toomey v Democrat Katie McGinty
Toomey is another Republican senator who was elected in the wave of 2010 at risk of losing his re-election. He is considered to be a moderate - and popular - Republican who was endorsed by Gabby Giffords pro-gun regulation group because of his support of gun laws. He even released an ad in the final weekend before Election Day featuring President Barack Obama praising Toomey for his support on gun control.
But Pennsylvania is a presidential battleground state and that race is having an impact on Toomey, who is down two points, according to the Real Clear Politics polling average. He has refused to say if he plans to vote for Trump, saying he "remains unpersuaded," giving fodder to his challenger, McGinty, who says that Toomey doesn't have the "courage" to stand up to Trump. Toomey has responded that he's running his own race but vows to "stand up" to Trump in Washington.
McGinty, a former Secretary of the Pennsylvania Department of Environmental Protection, has enjoyed consistent campaigning of presidential nominee Hillary Clinton and President Barack Obama who have campaigned with her in the presidential battle ground.
4). Indiana: Republican Rep. Todd Young v Democrat Evan Bayh
In this open seat to replace retiring Republican Sen. Dan Coats, Democrat Bayh is trying to win his seat back. He retired from the Senate in 2011 and with more than $10 million left over in his campaign account, Democrats recruited him to return to the Senate.
But Bayh, also a former governor and son of former senator Birch Bayh, has not been able to coast to what was expected to be an easy victory because of his 20-point lead earlier this year. But his challenger, Rep. Todd Young and allies have effectively painted him as a disconnected politician who has spent hardly any time in Indiana since he left the Senate six years ago and is more interested in making money and pleasing corporate interests than serving the people of Indiana.
Young, a former marine, has received enthusiastic support from conservative groups who have spent readily to help him. The IndyStar tabulated the amount spent on the Senate race by the candidates and outside groups has been more than $30 million through September, with Young receiving the slight advantage with the help from the main Senate Republican super PAC Senate Leadership Fund's $9 million worth of advertising in the state.
Young has endorsed Trump but threatened to renege on his support after the Access Hollywood video was leaked. But Young has not yet withdrawn his support in the conservative state that is likely to support Trump on November 8.
5). Missouri: Sen. Roy Blunt v Democrat Jason Kander
The Missouri Senate race was not expected to be close until Republican presidential candidate Donald Trump has made the political environment difficult for down-ballot candidates. Simultaneously, however, Kander has proven to be a strong campaigner, resulting in the tightening of polls in recent weeks with Blunt seeing only a one point advantage, making this one of the races that Democrats could win.
Blunt, who rose through the ranks in the House of Representatives and quickly became a part of leadership in his first term in the Senate, has endorsed Trump but has tried to run as an independent separate from the bombastic nominee. As the Kansas City Star notes, Kander has not focused on Blunt's support of Trump because Kander might need some of Trump's supporters if he wants to beat Blunt in the conservative state.
And Kander has had one of the most effective ads this campaign cycle. The telegenic 35-year old Missouri secretary of state assembles a rifle blindfolded, while saying, "Sen. Bunt has been attacking me on guns." He continues to detail his role in the U.S. Army and his positions on guns.
While Kander has sought to tie Blunt to Washington, and Blunt is trying to tie Kander to Hillary Clinton. What could be a damaging closing argument ad by the Senate Leadership Fund in the red state ties Kander to Clinton. "One Hillary in Washington would be bad enough," the narrator says.
6). Nevada: Rep. Joe Heck v Catherine Cortez Masto
Heck and Cortez Masto are running to fill Senate Minority Leader Harry Reid's open seat.
Cortez Masto, Sen. Reid's hand-picked successor and former Nevada attorney general, would be the first Latina to serve in the Senate if she won. She is aggressively trying to tie Heck to Trump, hoping that the large Latino population backs her.
Heck, a doctor and brigadier general in the U.S. Army, was first elected to Congress in 2010, in the Republican wave, but has been struggling to balance the shadow of Trump. He withdrew his endorsement of Trump after the Access Hollywood tape but Democrats are taking advantage of a leaked tape of Heck saying at a fundraiser, "I want to support him, I really do." Heck continues to say, according to CNN, that Trump "has got to change his tone."
Heck was consistently leading in the polls until mid-October when Cortez Masto is nearly tied, but showing the volatility of the race, Heck has a four point advantage according to a RCP average in the final week. A tremendous amount of money has been spent in the race with Democrats spending at least $37 million and Republicans at least $35 million in television advertisements alone - and that doesn't count the last two weeks of the election.
7). North Carolina: Republican Sen. Richard Burr v Deborah Ross
Of all the Republicans running in competitive Senate races this year, Burr is the only one who wasn't elected in 2010. He's served in the Senate for two terms and is on the Senate Intelligence committee. He's trying to fend off Ross, a former state representative and former head of the ACLU in the state, in a race that has unexpectedly become close.
Burr, who easily won his last re-election in 2010, didn't start campaigning in earnest until the Fall, forcing the Republican Party and allied groups to spend money to bolster him there as Ross started to close.
As for the Trump impact, Burr has not come to Trump's defense after the Access Hollywood tape was released but said he will vote for him. Burr is, however, trying to tie Ross to Clinton, especially after the new laptop was found 11 days before Election Day. Ross has stood by Clinton and has received the benefit of a large cadre of big-name Clinton surrogates campaigning for her in the Tar Heel State, including President Barack Obama, First Lady Michelle Obama and Sen. Elizabeth Warren.
The polls show the race tied, with the latest Real Clear Politics polling average showing Burr with 1 point advantage.
8). New Hampshire: Republican Sen. Kelly Ayotte v Gov. Maggie Hassan
Incumbent Sen. Ayotte, who was elected to her first term in 2010, has struggled to manage the Trump factor in her race. After months of refusing to say she supports him or doesn't, a Trump moment became her most high profile moment from the campaign. During a debate with Gov. Hassan, she bumbled through her answer to a question about Trump before finally saying Trump was a "role model" for children. She came out days later saying she "misspoke."
Gov. Hassan, who is serving her second term as governor of the Granite State, is now being more directly tied to questions about the top of the Democratic ticket. Shortly after the news broke about the new emails potentially related to the Clinton investigation, Ayotte released a statement saying Hassan has "made a political calculation" in her support of Clinton. And Ayotte released a new digital advertisement showing clips of Hassan refusing to say if Clinton is honest or trustworthy.
In the race that is showing Ayotte up by 2 1/2 points in the final week, Trump's and Clinton's shortcomings are going to be pivotal.
9). Florida: Republican Sen. Marco Rubio v Rep. Patrick Murphy
After a losing campaign for president, Rubio reversed himself and decided to run for re-election, upending a Senate race that Democrats felt they had a good chance of winning.
Rep. Murphy, who was elected to the House four years ago, has weaknesses that Rubio has capitalized on. Republicans have attacked him for inflating his resume and relying on his father's wealth and connections. Democrats stopped spending money in the expensive advertising state, leaving Murphy to mostly defend himself.
Murphy is trying to tie Rubio to Trump, whom he's' criticized but still endorses, which Democrats have seized on as proof that Rubio is politically malleable. But polls show that Rubio is withstanding the attacks.
The presidential race is likely to have a strong impact on the outcome in the Senate race. Trump can't win the presidency without winning Florida and Clinton and her surrogates are spending a large amount of time there - something Murphy is hoping to benefit from.