- Maine, Nevada, North Dakota, South Carolina and Virginia hold primary elections on Tuesday.
- In Nevada and North Dakota, voters will set the stage for challenges to Sens. Dean Heller and Heidi Heitkamp, two of the Senate's most vulnerable members.
- Primaries for competitive House races will also take place in Virginia, Nevada and Maine.
Voters from across the country will cast their primary election ballots Tuesday in five states that will help to determine control of both the House and Senate in November.
The contests have stakes for whether the GOP can keep control of both chambers of Congress in November. Here are the races to watch this week:
Vulnerable Republican Rep. Barbara Comstock seeks re-election in the state's affluent 10th District. The area west of Washington has leaned slightly more Democratic in recent presidential elections than the country as a whole, according to Cook Political Report's Partisan Voter Index.
Six Democrats aim to take down Comstock. The party's primary contenders have largely pushed for bipartisan solutions and common ground rather than engaging in a race to the left.
Former State Department official Alison Friedman had easily raised and spent more than the other Democratic contenders as of late May. State Sen. Jennifer Wexton has the endorsement of Democratic Gov. Ralph Northam. She also had more cash on hand than Friedman as of last month.
Other Democratic candidates include Army veteran Dan Helmer and former Department of Veterans Affairs official Lindsey Davis Stover.
Democrats are eyeing — but have a lower chance of competing in — Virginia's 7th District. The seat held by GOP Rep. Dave Brat favors Republicans. Brat was backed by the tea party in 2014 when he defeated House GOP Majority Leader Eric Cantor in that year's primary. Former CIA operative Abigail Spanberger and Marine Corps veteran Dan Ward will vie for the district's Democratic nomination Tuesday.
In Virginia's Senate race this year, incumbent Democrat Tim Kaine appears relatively safe. Three Republicans are running Tuesday to challenge Kaine, who was Hillary Clinton's choice for vice president.
Prince William County Supervisor Corey Stewart is a pro-President Donald 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.
In Nevada, voters will set the stage for one of the most important Senate elections of the year. Vulnerable Republican incumbent Dean Heller aims to hold on to his seat in a state that has voted for a Democrat in the last three presidential elections.
Flipping Heller's seat would help Democrats in their push to either take control of the Senate or prevent the GOP from expanding its 51 to 49 seat majority in November. Democrats face a tough task with 26 party members or independents who caucus with them up for re-election this year.
Only nine Republicans have to run for their seats again this year. Clinton won Nevada by about 2 percentage points in 2016, making the state one of the top targets for her party this year.
Democratic Rep. Jacky Rosen is expected to breeze through Tuesday's Democratic Senate primary in a crowded field. She first won election to the House in 2016.
The race to fill Rosen's 3rd District seat is also expected to be competitive. Philanthropist Susie Lee has easily led the fundraising race among Democrats. She is the favorite in Tuesday's primary.
On the GOP side, pro-Trump candidate Danny Tarkanian withdrew from challenging Heller to run for the seat. The son of famed former University of Nevada, Las Vegas basketball coach Jerry Tarkanian narrowly lost to Rosen in 2016.
Other Republican contenders include state Sen. Scott Hammond and former TV reporter Michelle Mortensen.
In North Dakota's Senate race, one of the Senate's most vulnerable Democrats will try to hold on to her seat. Sen. Heidi Heitkamp, who runs unopposed Tuesday, aims to win in a state Trump won by more than 30 points.
Republican Rep. Kevin Cramer is expected to coast to his party's nomination.
Heitkamp and Democrats running in red states have tried to promote bipartisan work in Congress. She has voted with Trump's position about 56 percent of the time, according to FiveThirtyEight. The percentage is the third highest among Senate Democrats.
The winner of Tuesday's GOP primary for Cramer's House seat will likely win in November. State Sen. Kelly Armstrong is considered the favorite in North Dakota's lone House race.
Maine will try out a new voting system on Tuesday. The state will deploy ranked choice voting, where voters list candidates by preference instead of choosing one.
Only one of Maine's two House races is considered competitive this year. Democrats will vie on Tuesday to challenge Republican Rep. Bruce Poliquin in the GOP-leaning 2nd District.
State Rep. Jared Golden leads the fundraising and spending race among Democrats. Small businessman Lucas St. Clair and former local elected official Craig Olson are also running for the nomination.
It is unlikely any House seats in South Carolina will flip from one party to another in November. Still, Tuesday could see an incumbent face a serious primary threat.
Republican Rep. Mark Sanford hopes to stave off a challenge Tuesday in South Carolina's 1st District. Businesswoman Katie Arrington has attacked the congressman for not doing enough to support Trump in Congress.
Sanford, the former South Carolina governor, famously claimed to be hiking the Appalachian Trail when he disappeared in June 2009. He was actually in Argentina visiting a woman with whom he had an extramarital affair.
In one campaign ad, Arrington tells voters: "It's time for Mark Sanford to take a hike — for real this time."