North Dakota's Democratic Sen. Heidi Heitkamp faces re-election in a state President Donald Trump won by more than 30 points in 2016. In Nevada, Republican Sen. Dean Heller will try to defend his seat in a state Democrat Hillary Clinton narrowly won in 2016.
Both elections are crucial in determining the Senate's party breakdown following November's midterm elections. Republicans aim to keep or expand their 51 to 49 seat majority in the chamber.
The GOP's ability to do so will have major implications for which proposals the chamber will pursue next year. It will also affect Trump's ability to get political and judicial nominees approved.
While the political environment appears to favor Democrats this year, the party faces an uphill battle to gain seats in the Senate. Twenty-six Democratic incumbents and independents who caucus with them face re-election this year. Only nine Republican seats are up for grabs this year.
While Heitkamp and Heller's seats are two of the main events this year, voters on Tuesday will also choose a Republican challenger to Sen. Tim Kaine, D-Va. Kaine is considered relatively safe in the blue-leaning state.
Heitkamp, a 62-year-old serving her first Senate term, is set to face Republican Rep. Kevin Cramer in November's election. The third-term congressman unanimously won the North Dakota GOP's endorsement in April.
The seat held by Heitkamp is one of the top Republican targets in this year's midterms. Trump won North Dakota with more than 60 percent of the vote in 2016.
Support for the president is already a major point of contention in the race.
The senator and Democrats running in red states have tried to highlight cooperation with Republicans. Heitkamp has voted with Trump's position about 56 percent of the time, according to FiveThirtyEight. The percentage is the third highest among Senate Democrats.
Most recently, the Democratic senator championed a bill to roll back regulations on all but the largest U.S. banks. She joined with Republican Senate colleagues to pass it. When Trump signed the bill last month, Heitkamp was featured prominently at a White House event.
Notably, the conservative Koch brothers political network recently unveiled digital ads thanking her for supporting the bank bill. The free market operation has traditionally supported Republicans but has broken with Trump's GOP after the president's decision to put steel and aluminum tariffs on Canada, Mexico and the European Union.
Republicans have repeatedly targeted Heitkamp for voting against the GOP tax overhaul that passed in December.
Cramer, 57, has served as North Dakota's only House member since 2013. He has voted with Trump's positions about 99 percent of the time. He has hit Heitkamp for not doing enough to support the president's agenda.
The race is expected to draw huge sums of money from donors and national political groups. As of late May, Heitkamp's campaign had more than $5 million on hand. That compares with just under $2 million for Cramer's campaign.
Public polling of the race has been limited. A Gravis survey in February found a 3-point advantage for Heitkamp. The lead fell within the poll's margin of error.
Heller hopes to keep his seat in a state Clinton won by about 2 percentage points. He is expected to easily win Tuesday's GOP primary.
First-term Rep. Jacky Rosen is the favorite in Tuesday's Democratic primary.
Heller, 58, avoided one possible hurdle when challenger Danny Tarkanian decided to run in a House election rather than face the senator. A bitter primary could have left Heller bruised entering the general election.
Trump in March urged Tarkanian — the son of famed University of Nevada, Las Vegas, basketball coach Jerry Tarkanian — not to run for Senate. The president said Heller "is doing a really good job."
Trump tweet: It would be great for the Republican Party of Nevada, and it's unity if good guy Danny Tarkanian would run for Congress and Dean Heller, who is doing a really good job, could run for Senate unopposed!
Heller has notably found himself in a tough spot on whether to repeal and replace the Affordable Care Act. Last year, the senator stood by Nevada Gov. Brian Sandoval, a fellow Republican, and said he would oppose repeal efforts if they hurt the Nevadans who received coverage under the law's Medicaid expansion. Trump later joked with Heller that he would support Obamacare repeal if he "wants to remain a senator."
Heller later voted for a bill to partly repeal the health-care law. It failed as GOP Sens. John McCain of Arizona, Susan Collins of Maine and Lisa Murkowski of Alaska voted against it.
In April, he pledged to overhaul the U.S. health-care system, according to the Las Vegas-Review Journal.
Rosen, 60, first got elected in 2016. She has voted with Trump's priorities only 36 percent of the time, according to FiveThirtyEight. That falls well short of Heller, who has voted with the president about 90 percent of the time.
Rosen's campaign has repeatedly targeted the Republican senator for his push to repeal Obamacare. She has also hit Heller for his vote in favor of the GOP tax plan, which she argues has enriched large corporations more than middle-class Americans.
As of mid-may, Rosen's campaign had both raised and spent more than Heller's operation. She had about $2.8 million on hand, trailing Heller's $4.9 million.
Public polling in the Nevada race has also been scarce. A Nevada Independent/Mellman poll in April found a 1-point edge for Heller. The advantage fell within the poll's margin of error.
Kaine appears safe for now in Virginia's Senate race in November. Recent statewide results bode well for the senator, who was Clinton's running mate in 2016.
Clinton won Virginia by about 5 percentage points in 2016. Then, Democratic Gov. Ralph Northam won his election last year by about 9 percentage points.
Three Republicans vie Tuesday for the chance to take on Kaine.
Prince William County Board of Supervisors Chairman Corey Stewart is a pro-Trump candidate who nearly won Virginia's GOP primary for governor last year. Virginia House of Delegates member Nick Freitas leans Libertarian and has received endorsements from Sens. Rand Paul, R-Ky., and Mike Lee, R-Utah.
The third candidate, pastor E.W. Jackson, was Virginia's Republican nominee for lieutenant governor in 2013.