WASHINGTON – While all eyes are on who could be the future occupant of the White House, the battle over control in both chambers of Congress is also heating up, especially as more lawmakers announce their retirement.

The retirements, most of which were announced by Republicans, open up a series of key races ahead of the 2020 elections as Republicans try to fend off Democrats aiming to take control of the Senate and maintain, or perhaps grow, their majority in the House.

Here is the list of lawmakers who have announced they aren't running to keep their seats in 2020.

House

So far, 30 members of the House have announced they won't be running in 2020 or have already left off: twenty-three Republicans and seven Democrats.

Among those leaving Congress are two of House Republicans' 13 women, including the female lawmaker that was tasked with recruiting more conservative women and minorities to the body.

Phil Roe: Republican representing Tennessee's 1st District

Roe, who has served in Congress for 11 years and is the top Republican on the House Veterans Affairs Committee, announced he would retire at the end of the 116th Congress.

"I’ll leave Congress at the end of the year knowing that our nation’s heroes are better served today because of our work," he said.

Roe's district in central Tennessee has been a safe Republican seat for decades.

Mark Meadows: Republican representing North Carolina's 11th District

Meadows declared he would not run again the day after President Donald Trump was impeached by the House.

The former chairman of the conservative Freedom Caucus and staunch Trump ally said in a statement that his work with the president's administration "is only beginning," but he did not offer specifics.

"This President has accomplished incredible results for the country in just three years, and I’m fully committed to staying in the fight with him and his team to build on those successes and deliver on his promises for the years to come," Meadows said

Duncan Hunter: Republican representing California's 50th District

Hunter announced he would be resigning from Congress "shortly after the holidays" after pleading guilty in federal court to misusing campaign funds.

"It has been an honor to serve the people of California's 50th District, and I greatly appreciate the trust they have put in me over these last 11 years," the Republican lawmaker said in a statement.

Hunter and his wife were indicted on 60 counts of campaign finance violations of spending $250,000 in campaign funds in personal usage by federal prosecutors in August 2018.

The district in Southern California is traditionally Republican.

Hunter won re-election by 3 points in 2018, despite facing the indictment.

George Holding: Republican representing North Carolina's 2nd District

Holding, who has served in Congress since 2013, announced his retirement in a Dec. 6 statement, citing the heavy Democratic lean of his new congressional district among the reasons for his retirement.

"I should add, candidly, that, yes, the newly redrawn Congressional Districts were part of the reason I have decided not to seek reelection," he said in a statement.

Holding's Raleigh-area district was redrawn after a federal court ruled North Carolina's congressional districts were illegally gerrymandered.

Tom Graves: Republican representing Georgia's 14th District

Graves, who was first elected to Congress in 2010, announced in a Dec. 5 tweet he would be retiring from Congress.

In a letter to constituents, the member of the House Appropriations Committee said he had entered a "new season in life" and would not be seeking re-election.

The heavily Republican district in northwest Georgia is likely to remain in Republican control.

Denny Heck: Democrat representing Washington's 10th District

Rep. Denny Heck, who has been in Congress since 2013, announced in a Dec. 4 tweet and statement posted to Medium that he would retire after 2020.

He bemoaned the state of public discourse, writing "There are simply too many hyperbolic adjectives and too few nouns. Civility is out. Compromise is out. All or nothing is in."

Heck has played a key role in the impeachment inquiry into President Donald Trump as a member of the House Intelligence Committee.

His Olympia, Washington-area district is likely to remain in Democratic hands.

Pete Visclosky: Democrat representing Indiana's 1st District

Rep. Pete Visclosky, who has been in office since 1985, announced in a Nov. 6 tweet and a statement that he would not be running for re-election in 2020.

"I thank the citizens of the First District for their trust and for providing me the incredible life privilege of serving you in the United States Congress," he said in a statement.

Walden sits on the House Appropriations Committee and chairs its subcommittee on Defense.

His heavily Democratic Gary, Indiana-area district is likely to stay in Democratic hands.

Greg Walden: Republican representing Oregon’s 2nd District

Rep. Greg Walden, who has been in office since 1998 and is Oregon’s only Republican representative, announced that he would not seek re-election after his current term ends in a video statement Oct. 28. He said he will not run for public office again.

“For me, the time has come to pursue new challenges and new opportunities,” Walden said. “I will close the public service chapter of my life, thankful for the friends that I’ve made and the successful work that we’ve done together.”

Walden serves on the House Energy and Commerce Committee and was its chairman from 2017 through the beginning of this year.

Francis Rooney: Republican representing Florida's 19th district

Rep. Francis Rooney of Florida's 19th district announced on Oct. 19 in a Fox News interview that he would not seek re-election in 2020.

Rooney, who has served in Congress since 2017 and also was the Ambassador to the Holy See under the George W. Bush administration, had been critical of Trump.

The deeply conservative district is likely to remain in Republican hands.

Nita Lowey: Democrat representing New York's 17th district

Rep. Nita Lowey of New York's 17th district announced on Oct. 10 she would not seek re-election in 2020.

Lowey, who was first elected to Congress in 1988, is also the first chairwoman of the powerful House Appropriations Committee.

“After 31 years in the United States Congress, representing the people of Westchester, Rockland, Queens and the Bronx, I have decided not to seek re-election in 2020," she said in a statement.

The progressive district is likely to remain in Democratic hands.

Chris Collins: Republican representing New York’s reddest district

Rep. Chris Collins of New York's 27th district announced he was resigning from Congress prior to pleading guilty on criminal insider-trading charges.

His resignation became official when it was read into the record on the House of Representatives floor Tuesday morning.

Collins, a fourth-term congressman, was the first sitting member of Congress to endorse Donald Trump for president in the 2016 race, and narrowly won re-election last November despite being under indictment.

Mac Thornberry: Texas Republican representing the state's 13th District.

Rep. Mac Thornberry, a former chairman of the Armed Services Committee, said he would not seek reelection in 2020.

Thornberry has been a member of Congress since 1994. His district in the Texas Panhandle has been safely Republican and isn't considered competitive.

Paul Cook: Republican representing California's 8th District

Rep. Paul Cook, who has been representing his California district since 2013 and is currently in his fourth term, announced Tuesday that he will not seek re-election again in 2020.

"Serving in Congress has been an absolute honor, and I'm proud of my numerous victories amid a tough partisan atmosphere. Our high desert needs continued strong leadership at the county level, and I pledge to fight for this area with the same dedication and conviction I've demonstrated my entire career," he said in a statement, according to multiple media reports.

Cook won re-election in 2018 with 60% of the vote.

He said he will pursue a seat on the San Bernardino County Board of Supervisors.

Jim Sensenbrenner: Republican representing Wisconsin's 5th District

Rep. Jim Sensenbrenner, who is the second-longest serving member of the House of Representatives, announced on Wednesday that he would not run again in 2020 for a 22nd term in Congress.

“I think the time has come to basically turn over the page in the 5th District,” he said in an interview with the Milwaukee Journal-Sentinel.

His solidly Republican district of suburban, exurban and rural communities north and west of Milwaukee County is likely to remain in Republican control.

Susan Davis: Democrat representing California's 53rd District

Rep. Susan Davis, who has represented the solidly blue southern California district for nearly 20 years, announced on Wednesday that she would not run again in 2020.

"I am immensely grateful to you for the repeated confidence you have shown in me," she said in a letter to constituents. "I have struggled to make this very difficult decision. I will not seek another term in Congress."

Bill Flores: Republican representing Texas' 17th District

Rep. Bill Flores announced September 4 that he did not intend to run for re-election in 2020.

"After much prayer over the past few days and following conversations with my wife, Gina, during that time, I have decided that my current term will be my last," Flores said in a statement, also noting that he intended to return to the private sector and remain politically active.

Flores has represented this Waco-area district since 2011, and won about 57% of the vote in 2018 in the reliably Republican district.

John Shimkus: Republican representing Illinois' 15th District

Rep. John Shimkus announced on Aug. 30 that he would not run for re-election in 2020. He announced his decision on KMOX radio in St. Louis. He said in a statement that he was looking forward to his "next chapter of life."

Shimkus, who has represented the district since 2003, won about 70 percent of the vote in 2018 in a solidly red district, which Donald Trump won in 2016.

Sean Duffy: Republican representing Wisconsin's 7th District

Rep. Sean Duffy announced Aug. 26 he would be leaving Congress in September.

More:GOP Congressman Sean Duffy says he's stepping down due to new baby

Duffy, who has represented his district in northern Wisconsin since 2011, said in a Facebook post that he needs to step down in order to devote more time to a baby he and his wife are expecting in October that has a heart condition. Duffy's was once a true swing seat in Congress but was made more Republican in 2011 by GOP redistricting, and has also shifted right as rural areas continue to tilt toward Republicans.

Kenny Marchant: Republican representing Texas' 24th District

Rep. Kenny Marchant, an eight-term veteran, announced he wouldn't run for re-election on Aug. 5. Marchant, 68, was re-elected by a 3 percentage-point margin last year from his suburban district between Dallas and Fort Worth. He'd won by 17 percentage points in 2016 and by 33 percentage points in 2014.

More:7th Republican representative in two weeks says he won't run for re-election

"I am looking forward to finishing out my term and then returning to Texas to start a new chapter," Marchant said in a statement.

Will Hurd Republican representing Texas' 23rd District

Rep. Will Hurd, the lone Black Republican in the House and a strong critic of President Donald Trump, announced Aug. 1 that he will not seek re-election. In 2018, Hurd won a very slim victory — less than 1,000 votes — in his western Texas district.

More:Will Hurd, lone Black House Republican, won't seek reelection

"I have made the decision to not seek reelection for the 23rd Congressional District of Texas in order to pursue opportunities outside the halls of Congress to solve problems at the nexus between technology and national security," Hurd wrote on Twitter.

Mike Conaway Republican representing Texas' 11th District

Rep. Michael Conaway announced July 31 that he won’t seek a ninth term representing a sprawling West Texas congressional district.

Conaway announced his decision at a news conference in Midland. In a statement, he said that while serving in the House, he had asked his family "to make innumerable sacrifices." He said the time had come for him to put his family first.

Martha Roby Republican representing Alabama's 2nd District

Rep. Martha Roby, who has represented much of Montgomery and southeast Alabama in the House of Representatives since 2011, said July 26 that she will not run for re-election.

More:U.S. Rep. Martha Roby won't seek re-election

Roby did not specify a reason for her departure from Congress in a statement emailed and posted on Twitter, saying that she and her family "will be forever grateful to the people of AL-02 for giving us the tremendous privilege & honor of serving our state & country."

Pete Olson Republican representing Texas' 22nd District

Rep. Pete Olson said July 25 he won’t seek re-election in 2020, giving up his House seat that Democrats were already targeting for next year.

Olson said he’ll retire after his sixth term to “be a more consistent presence” with family. He narrowly won re-election in 2018 in his suburban Houston district.

Paul Mitchell Republican representing Michigan's 10th District

Rep. Paul Mitchell, a wealthy businessman who spent millions of his own money to win a seat in Congress, said July 24 that he will step down after just two terms.

More:Rep. Paul Mitchell won't run again, complains of 'rhetoric and vitriol' in Washington

Mitchell, who replaced former Rep. Candice Miller, R-Harrison Township, in 2017, after she stepped down, represents a safely Republican district, which includes parts of Macomb County and the Thumb.

Susan Brooks Republican representing Indiana's 5th District

Rep. Susan Brooks, one of only 13 Republican women in the House as well as the head of GOP recruitment for 2020, announced she would not run for re-election in an exclusive interview with USA TODAY on June 14.

More:Exclusive: GOP lawmaker who sought to recruit more women to run in 2020 is herself retiring

“While it may not be time for the party, it’s time for me personally,” Brooks, 58, said. Democrats have been eyeing her district, which includes the wealthy northern Indianapolis suburban areas, as potentially flippable as Republican support has eroded in some suburban areas under Trump.

Dave Loebsack Democrat representing Iowa's 2nd District

Rep. Dave Loebsack announced April 12 that he will retire at the end of this term, after representing Iowa in Congress for 14 years.

More:Democratic Congressman Dave Loebsack to retire after this term

The Iowa City Democrat, 66, who represents the southeast quarter of the state, was first elected to the House in 2006. He spent part of his career as the state's sole Democrat in either the U.S. House or Senate. "I have enjoyed beyond my expectations serving the people of Iowa’s Second District for the past 13 years," Loebsack said.

Jose Serrano Democrat representing New York's 15th District

Rep. Jose Serrano, a 16-term Democrat from the South Bronx, announced on March 25 that he has Parkinson’s disease and will retire at the end of his term.

The 75-year-old is a fixture in Bronx politics and is among Congress’ foremost defenders of Puerto Rico, the U.S. territory where he was born. First elected in 1990, Serrano is the most senior Latino currently serving in Congress.

Rob Woodall Republican representing Georgia's 7th District

Rep. Rob Woodall, who barely escaped defeat last year in a suburban Atlanta seat once considered safe for GOP candidates, announced Feb. 7 that he won’t seek re-election in 2020.

Woodall’s district was one of two Georgia congressional seats targeted in the 2018 midterms by Democrats. He won his fifth term by fewer than 450 votes. "I have realized over this past year of change—both in politics and in my family—that the time has come for me to pass the baton and move to the next chapter," Woodall said in a statement.

Rob Bishop Republican representing Utah's 1st District

Rep. Rob Bishop announced back in Aug. 2017 that he planned to retire and not run again in 2020. First elected in 2002 to the heavily red Utah district, Bishop plans to retire at the end of his current term, when his service in committee leadership expires under GOP rules.

He has served as chairman of the powerful House Natural Resources Committee, and is now its ranking member.

Katie Hill: Democrat representing California's 25th District

Rep. Katie Hill, faced with allegations she had a sexual relationship with a staff member, announced her resignation on Oct. 27, less than a year after unseating a Republican incumbent in a purple district near Los Angeles.

In a statement posted to Twitter, Hill called her resignation "the hardest thing I have ever had to do, but I believe it is the best thing for my constituents, my community, and our country."

The House Ethics Committee announced days earlier that it had launched an investigation into allegations Hill engaged in a sexual relationship with a staff member. Hill said claims that she was in a relationship with her legislative director are "absolutely false."

Senate

Five members of the Senate have announced they won't run for re-election in 2020, including four Republicans.

Democrats are hoping to take control of the chamber as they did with the House in 2018. In the midterms, though, Senate Republicans were not only able to fend off Democrats, they also picked up two seats.

But the 2020 election will differ from the midterms as the president will be on the ballot. Voter sentiment about Trump is likely to play a bigger role in determining who turns out at the polls and which party they support.

In 2020, Democrats need to gain four seats, only three if they take the White House. Twelve Democrats and 22 Republicans are up for re-election in 2020. Many of the GOP seats are in red states that previously voted for Trump but the retirement announcements could help in a number of key races.

Johnny Isakson Republican representing Georgia

Sen. Johnny Isakson announced Aug. 28 that he will retire at year's end, a departure that sets up a rare election in November 2020 when both of the state's Senate seats will be on the ballot and teeing up what could become a battle for Republicans to retain the state.

The 74-year-old lawmaker said he is leaving the job he loves because "mounting" health issues, including Parkinson's disease, are "taking their toll" on his work, family and staff.

Mike Enzi Republican representing Wyoming

Sen. Mike Enzi announced on May 4 that he would not run for a fifth term in 2020.

Enzi, 75, announced his pending retirement in his hometown of Gillette, where he owned a shoe store and “never intended to get into politics.” With Enzi’s retirement, Wyoming will have its first open Senate seat in more than a decade, though it’s expected to remain in Republican hands.

Tom Udall Democrat representing New Mexico

Sen. Tom Udall announced March 25 that he would not seek re-election in 2020, though the seat is favored to remain in Democratic control.

Udall said he believes he could win another term “but the worst thing anyone in public office can do is believe that the office belongs to them, rather than to the people they represent."

Pat Roberts Republican representing Kansas

Sen. Pat Roberts, the longest-serving member of Congress in Kansas history, announced on Jan. 4 that he won’t run again in 2020, setting up a scramble to replace him in a GOP-leaning state where Democrats are energized by key victories in last year’s midterm elections.

The 82-year-old, four-term senator was likely to have faced grueling primary and general election contests next year.

Lamar Alexander Republican representing Tennessee

After roughly a quarter-century in elected office, Sen. Lamar Alexander will retire in 2020.

More:Tennessee Sen. Lamar Alexander will not seek re-election in 2020

The former Republican governor, who has served in the Senate since first being elected in 2002, announced in December 2018 that he will not seek a fourth term in the upper chamber. Alexander is chairman of the key Senate Committee on Health, Education, Labor, and Pensions, which handles everything from education policy to issues with the Affordable Care Act.

Contributing: William Cummings, Savannah Behrmann, Rebecca Morin, Ledyard King and Nicholas Wu of USA TODAY, Brian Lyman of the Montgomery Advertiser, Todd Spangler of the Detroit Free Press, Craig Gilbert and Molly Beck of the Milwaukee Journal Sentinel and the Associated Press