Republicans hold the majority in the U.S. House – but Democrats are hoping to ride a “blue wave” in the midterm elections to take back control.

There are dozens of House races in the midst of contentious and close battles across the country that could make or break it for Democrats this year – including several GOP-held seats in California that are no longer a shoo-in for the Republican Party.

Read on for a look at 20 House to keep your eye on during the midterm elections.

Arizona 2: Ann Kirkpatrick vs. Lea Marquez Peterson

With incumbent Rep. Martha McSally leaving her seat in a bid for the U.S. Senate, Fox News has ranked the race to replace her in Arizona’s 2nd congressional district as leaning Democrat.

Democrat Ann Kirkpatrick, a former congresswoman, previously ran against the late Sen. John McCain in 2016. She now faces Republican Lea Marquez Peterson, the president & CEO of the Tucson Hispanic Chamber of Commerce, in the congressional race.

California 10: Jeff Denham vs. Josh Harder

Fox News has ranked California’s 10th congressional district a toss-up as GOP Rep. Jeff Denham fights to hang onto his seat. He faces Josh Harder, a venture capitalist and business professor, in the election.

Hillary Clinton won the district in 2016, gearing Democrats up for a fight to challenge Denham, who has frequently struggled in this re-election campaigns, according to KTXL-TV.

California 21: TJ Cox vs. David Valadao

As Rep. David Valadao fights to hang onto his congressional seat, Fox News has ranked the race as leaning Republican. Valadao faces Democrat TJ Cox, an engineer and businessman, in the race for California’s 21st congressional district.

Clinton won the district in 2016.

Cox was previously criticized for claiming a nearly $1 million home in Maryland as his primary residence.

California 25: Katie Hill vs. Steve Knight

Katie Hill, who once led a large California nonprofit to help the homeless, is hoping to unseat two-term incumbent GOP Rep. Steve Knight in California’s 25th congressional district. Fox News has ranked the race a toss-up.

Knight, an Army veteran, served in the California state House before being elected to Congress. Clinton won the district in the 2016 presidential election.

California 39: Gil Cisneros vs. Young Kim

The retirement of longtime Rep. Ed Royce, a Republican, set up a contentious battle for California’s 39th congressional seat. Fox News has ranked the race a toss-up.

Democrat Gil Cisneros, a Navy veteran and education advocate, faces Republican Young Kim, a former state legislator, in the election. Kim has touted the fact she was the first Korean-American GOP woman elected to California’s state Assembly.

California 45: Katie Porter vs. Mimi Walters

Finishing her second term in Congress, GOP Rep. Mimi Walters faces Katie Porter in a tough re-election battle that Fox News has ranked as a toss-up.

Porter is a consumer protection attorney who also teaches law at the University of California, Irvine. A former longtime state lawmaker, Walters has nearly consistently voted in agreement with President Trump since she’s been in Congress.

California 48: Dana Rohrabacher vs. Harley Rouda

Longtime GOP Rep. Dana Rohrabacher has been a polarizing figure in Congress, where he’s served since 1989. Although it endorsed him, Rohrabacher’s hometown newspaper said he “has a tendency to say wildly inappropriate things” and has “an affinity for Vladimir Putin.”

Rohrabacher faces Harley Rouda, an Orange County businessman, in the race which Fox News has ranked a toss-up. The election is one of the nation’s most costly this year, which former New York City Mayor Michael Bloomberg spending more than $4 million in support of Rouda.

California 49: Diane Harkey vs. Mike Levin

With Rep. Darrell Issa’s retirement, the race for California’s 49th district has come down to Republican Diane Harkey and Democrat Mike Levin.

Harkey is the chair of the State Board of Equalization – which “administers property tax, alcoholic beverage tax, and tax on insurers programs,” according to its website – and is a former state lawmaker. Levin is a community organizer who has focused on climate change throughout the race.

Fox News has ranked the race as leaning Democrat.

California 50: Ammar Campa-Najjar vs. Duncan Hunter

Embattled Rep. Duncan Hunter is locked in a tough re-election fight against Democrat Ammar Campa-Najjar. Fox News has ranked the race as leaning Republican.

Republicans have harped on Campa-Najjar’s family throughout the race, and Hunter has said he is a “national security risk” because of his ties to Palestine and his grandfather’s involvement in the 1972 Olympics terrorist attack, The San Diego Tribune reported.

Campa-Najjar, who was granted a federal security clearance when he worked in the Obama administration, has admitted his grandfather “was associated with something that was heinous, that was wrong and he was brought to justice.” He also disavowed “the murder of innocent people.”

Hunter, too, isn’t without his controversies. He and his wife were indicted earlier this year on charges of illegally converting $250,000 in campaign funds for personal expenses and filing false records, prosecutors said. Hunter has brushed off the allegations as “pure politics.”

Florida 27: Maria Elvira Salazar vs. Donna Shalala

It’s a district that Democrat Clinton won by a 20-point margin over Trump in 2016 but still, Fox News has it ranked a toss-up as Donna Shalala has seemed to lose momentum ahead of Election Day.

Shalala is the former Health and Human Services secretary under the Clinton administration and president of the University of Miami. She faces Republican Maria Elvira Salazar, a Spanish-language journalist, in the race to replace retiring GOP Rep. Ileana Ros-Lehtinen.

Iowa 1: Rod Blum vs. Abby Finkenauer

If Democrat Abby Finkenauer can defeat incumbent Rep. Rod Blum in Iowa’s 1st congressional district, she would be the youngest member of Congress at just 29 years old. Fox News has ranked the race as leaning Democrat.

The district, which features Iowa’s northern suburbs and more than a dozen college campuses, delivered double-digit wins for former President Barack Obama twice before breaking for Trump by three over Clinton.

Kansas 2: Paul Davis vs. Steve Watkins

Ranked a toss-up by Fox News, the race between Democrat Paul Davis and Republican Steve Watkins is a close one.

Davis, a former Kansas House leader, and Watkins, an Army veteran and political newcomer, are seeking the seat held by retiring GOP Rep. Lynn Jenkins.

Kentucky 6: Andy Barr vs. Amy McGrath

In one of the most expensive and competitive races in the country, Republican Rep. Andy Barr is fighting to hold onto his seat, facing a tough challenger in Amy McGrath, a retired Marine.

Fox News has ranked the race for Kentucky’s 6th congressional district a toss-up. McGrath, who would be the first woman to hold the seat if elected, went viral with her 2-minute long campaign video about her life earlier this year.

Michigan 8: Mike Bishop vs. Elissa Slotkin

Two-term Rep. Mike Bishop, a Republican, faces Democrat Elissa Slotkin in the battle for Michigan’s 8th congressional district, which Fox News has ranked as a toss-up.

Slotkin is a former CIA analyst who has also worked for the White House and Departments of State and Defense in both the Bush and Obama administrations.

A familiar name to his district, Bishop was a longtime state lawmaker, eventually serving as the minority leader of the Michigan state Senate, before his election to Congress.

Minnesota 8: Joe Radinovich vs. Pete Stauber

Minnesota’s 8th congressional district went for Trump in 2016, and Republicans have had their eye on flipping the seat, especially after incumbent Rep. Rick Nolan announced he wouldn’t run for re-election.

Fox News has ranked the race between Joe Radinovich, a former state lawmaker, and Pete Stauber, a former county commissioner and veteran, as leaning Republican. Except for a brief, two-year period in 2010, the seat has remained steadfastly blue since the late 1940s.

New Hampshire 1: Eddie Edwards vs. Chris Pappas

It’s an election poised to make history no matter who wins New Hampshire’s 1st congressional district. Republican Eddie Edwards, a Navy veteran and former police chief, would be New Hampshire’s first black congressman. And Democrat Chris Pappas, a former state lawmaker, would be the state’s first openly gay congressman.

Fox News has ranked the race as leaning Democrat. The district went for Trump in the 2016 election. The seat is open as incumbent Rep. Carol Shea-Porter, a Democrat, is retiring at the end of this term.

New Jersey 11: Mikie Sherrill vs. Jay Webber

With the retirement of venerable Republican Rep. Rodney Frelinghuysen, the race for New Jersey’s 11th congressional district has been ranked by Fox News as leaning Democrat.

Mikie Sherrill is the Democratic nominee in the race. She is a Navy veteran and federal prosecutor. Webber, a Republican, is a member of the New Jersey state Assembly.

Texas 32: Colin Allred vs. Pete Sessions

Republican Rep. Pete Sessions has been in Congress since 1997, representing what is now the 32nd congressional district since 2003. But the longtime congressman’s seat appears to be in jeopardy this year as he’s locked in an expensive re-election campaign against Democrat Colin Allred.

Fox News has ranked the race as a toss-up. Sessions didn’t face a Democratic challenger in 2016 – but his district turned blue for Clinton over Trump.

A civil rights attorney from Dallas, Allred said he was “inspired” by the “many grassroots voices” that have emerged since the 2016 election. He is also a former NFL player who played for the Tennessee Titans. He’s drawn on his upbringing – raised by a single mother and their community – throughout his campaign, according to the Dallas News.

Virginia 10: Barbara Comstock vs. Jennifer Wexton

Rep. Barbara Comstock was first elected to represent Virginia’s 10th congressional district in the U.S. House in 2014. She won re-election once in 2016, but this year her seat is in jeopardy as she’s considered one of the most vulnerable Republicans in Congress.

Fox News has ranked the race between Comstock and state Sen. Jennifer Wexton, a former prosecutor, as leaning Democrat. A Republican has represented the district for more than three decades.

Wisconsin 1: Randy Bryce vs. Bryan Steil

It’s House Speaker Paul Ryan’s seat – which he’s held since for nearly two decades – but Fox News has only ranked it as leaning Republican.

Bryan Steil, a manufacturing sector attorney, is the Republican vying to take over his former boss’ seat. Bryce, an Army veteran and union ironworker, is the Democrat.

Fox News’ Paulina Dedaj, Peter Doocy, Chad Pergram and Allie Raffa contributed to this report.