For some Republicans, it’s starting to feel like 2006 — a wave election year that swept Democrats back into power in the House and Senate.

The retirement of two longtime California Republicans this week — just the latest in a string of House Republicans heading for the exits — has caused panic among some in the GOP who say it’s yet another sign that an anti-Trump, Democratic wave is forming.

“It’s a tough election cycle for Republicans; we know that going in,” said Rep. Ileana Ros-Lehtinen Ileana Carmen Ros-Lehtinen'Trump show' convention sparks little interest on K Street Shalala to face Salazar in Florida rematch TechNet hires Hispanic communications director MORE (R-Fla.), who is not running for reelection after representing a heavily Hispanic Miami district for nearly 30 years.

“It’s starting to feel very scary for moderate Republicans,” she said.

Rep. Darrell Issa Darrell Edward IssaDCCC reserves new ad buys in competitive districts, adds new members to 'Red to Blue' program Wife of former Rep. Duncan Hunter sentenced to 8 months of home confinement Harris endorses Democrat in tight California House race MORE, who won reelection by a slim 1,621-vote margin in 2016, said Wednesday this term would be his last, despite insisting for months that he was running for reelection.

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The stunning announcement from the former Oversight and Government Reform Committee chairman came just two days after another veteran Republican from Southern California, Foreign Affairs Chairman Ed Royce Edward (Ed) Randall RoyceThe 'extraordinary rendition' of a US Presidential Medal of Freedom recipient, 'Hotel Rwanda' hero Gil Cisneros to face Young Kim in rematch of 2018 House race in California The most expensive congressional races of the last decade MORE, also called it quits.

Asked for his reaction to Issa’s retirement, Democratic Whip Steny Hoyer Steny Hamilton HoyerHouse Democrats postpone vote on marijuana decriminalization bill Democrats scramble on COVID-19 relief amid division, Trump surprise The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep MORE (Md.) paused, smiled, then exclaimed: “We’re gonna win the House back!”

The pair of retirements in California has altered the 2018 midterms landscape, forcing the House GOP’s campaign arm to decide whether it will defend two districts that overwhelmingly voted for Democrat Hillary Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE in 2016 or shift resources elsewhere.

Winning both districts could be costly. San Francisco billionaire Tom Steyer, who’s led a campaign to impeach President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE, this week pledged $30 million to help Democrats take back the House and said he would specifically target Issa.

Republicans have other reasons to be worried about the elections, including Trump’s approval rating, which sits in the high 30s.

History shows that a president’s party typically loses an average of 32 House seats during a midterm election. But Ros-Lehtinen said Trump might be a bigger liability than past GOP presidents in many parts of the country.

“In many districts like Darrell’s and mine, having President Trump an ever-present figure is a drag on the ticket,” she said. “In many districts, he’s a positive, but in districts like mine, it doesn’t help the Republican candidate.

“The Trump symbol, the Trump brand and Mr. Trump himself is a drag on moderate districts.”

The wave of GOP retirements in competitive districts also has set off alarm bells among some senior Republican strategists.

“I’m alarmed, but we should have already been alarmed. It’s a tough environment, and there’s a chance the Republicans can lose control of the House,” said Scott Jennings, a GOP political strategist who has worked on Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE’s (R-Ky.) reelection campaigns.

“It’s starting to feel like 2006 to me,” he added, “which was a bad year for Republicans.”

Democrats picked up 31 House seats in 2006, a victory that propelled them forward to win complete control of Washington in 2008.

This year, House Democrats need to flip 24 GOP-held seats to win back the majority. And the path to that new majority runs right through Orange County and San Diego, where traditional Republican districts like Royce’s and Issa’s have been getting more diverse and trending bluer.

Other top Democratic targets in Southern California include GOP Reps. Mimi Walters, Steve Knight and Dana Rohrabacher Dana Tyrone RohrabacherDemocrat Harley Rouda advances in California House primary Lawyers to seek asylum for Assange in France: report Rohrabacher tells Yahoo he discussed pardon with Assange for proof Russia didn't hack DNC email MORE, a lawmaker whose ties to Russia are receiving extra scrutiny amid the investigations into 2016 election meddling.

“You can’t hold this majority if you lose California districts because California districts look like suburban Pennsylvania districts and New Jersey [swing] districts,” explained Rep. Greg Walden Gregory (Greg) Paul WaldenTrump order on drug prices faces long road to finish line Ignore the misinformation: The FDA will ensure the safety of any COVID-19 vaccine Hillicon Valley: Trump backs potential Microsoft, TikTok deal, sets September deadline | House Republicans request classified TikTok briefing | Facebook labels manipulated Pelosi video MORE (R-Ore.), who served as chairman of the National Republican Congressional Committee (NRCC) during the successful 2014 and 2016 cycles.

“It’s a big concern,” a GOP aide said of the pair of California retirements. “These Orange County seats are majority makers.

“I hope [Rohrabacher] retires,” the aide added. “That’s a seat that can be held.”

But other retirements certainly aren’t helping the GOP. In addition to Issa, Royce and Ros-Lehtinen, moderate Reps. Charlie Dent Charles (Charlie) Wieder DentRepublican former Michigan governor says he's voting for Biden Biden picks up endorsements from nearly 100 Republicans Bush endorsing Biden? Don't hold your breath MORE (R-Pa.), Dave Trott David Alan TrottFormer GOP Michigan congressman says Trump is unfit for office Pro-Trump Republican immigrant to challenge Dem lawmaker who flipped Michigan seat Meet the lawmakers putting politics aside to save our climate MORE (R-Mich.), Dave Reichert David (Dave) George ReichertWashington Rep. Kim Schrier wins primary Mail ballot surge places Postal Service under spotlight Bottom Line MORE (R-Wash.) and Frank LoBiondo Frank Alo LoBiondoVan Drew wins GOP primary in New Jersey Amy Kennedy wins NJ primary to face GOP's Van Drew Stimulus price tag of .2T falls way short, some experts say MORE (R-N.J.) are not seeking reelection, providing more pick-up opportunities for Democrats.

The Cook Political Report, a campaign handicapper in Washington, moved Royce’s seat from “lean Republican” to “lean Democratic” this week; it moved Issa’s seat from “toss up” to “lean Democratic.”

“If you’re NRCC chairman, the last thing you want is a retirement in almost every case,” Walden told The Hill.

But he added that retirements sometimes allow a party to recruit a strong candidate who doesn’t have the political baggage of a veteran lawmaker.

“It does allow a reset,” Walden said.

Walden and many other senior Republicans insist they aren’t panicking, despite the fresh warning signs. Because of 2010 redistricting, most congressional districts are “baked in,” drawn in a way that favors either Republicans or Democrats, resulting in fewer swing districts than in past decades.

Speaker Paul Ryan Paul Davis RyanKenosha will be a good bellwether in 2020 At indoor rally, Pence says election runs through Wisconsin Juan Williams: Breaking down the debates MORE (R-Wis.) and other Republicans believe they have a winning campaign message in 2018, as most Americans will see a boost in their paychecks and lower tax bills following the historic passage last month of the tax overhaul. Republicans are also touting a slew of regulatory reforms and the confirmation of Supreme Court Justice Neil Gorsuch.

“I think it’s more likely that the House would change majorities than the Senate, given the map,” said Rep. Kevin Cramer Kevin John CramerOn Paycheck Protection Program, streamlined forgiveness is key McConnell shores up GOP support for coronavirus package Army Corps urges DOJ to settle case with ND over M DAPL damages MORE (R-N.D.), “but we have also done some good things that we can campaign on, and hopefully we will do some good things [this year] that we will continue to campaign on.”

Jennings, the GOP strategist, said he has a high degree of confidence in Ryan and the campaign team led by current NRCC Chairman Steve Stivers Steven (Steve) Ernst StiversBusiness groups back pandemic insurance bill modeled on post-9/11 law National Retail Federation hosts virtual 'store tours' for lawmakers amid coronavirus Stronger patent rights would help promote US technological leadership MORE (R-Ohio), especially given their big legislative victory on taxes.

Two Ryan-aligned super PACs — the American Action Network and Congressional Leadership Fund — said they raised a record $66 million in 2017, helping the latter to open offices in 27 GOP-held districts this cycle, including in California.

“I know the political team around Speaker Ryan has been anticipating open seats in tough districts. I don’t think anyone is caught flat-footed,” Jennings said. “But what I’m worried about is the macro conditions that appear to be lining up against Republicans.”

“The Republicans can hang on ... but it’s gonna take a lot of focus and smart campaigns,” he added.

Some GOP sources familiar with the NRCC’s operation are conceding the party could lose as many as 15 seats this fall, but that would still keep the House in Republican hands.

The wild card, of course, is whether any more vulnerable Republicans decide they’ve had enough of Congress. Other long-serving Republicans who’ve landed on Democrats’ retirement watch list include former Energy Chairman Fred Upton Frederick (Fred) Stephen UptonOn Paycheck Protection Program, streamlined forgiveness is key Hispanic Caucus campaign arm endorses slate of non-Hispanic candidates Progressives soaring after big primary night MORE (R-Mich.), Appropriations Chairman Rodney Frelinghuysen Rodney Procter FrelinghuysenBottom line Republican lobbying firms riding high despite uncertainty of 2020 race Ex-Rep. Frelinghuysen joins law and lobby firm MORE (R-N.J.), Rules Chairman Pete Sessions Peter Anderson SessionsThe Hill's Campaign Report: New polls show Biden leading by landslide margins The Hill's Morning Report - Presented by Argentum - In Rose Garden, Trump launches anti-Biden screed Pete Sessions wins GOP runoff in comeback bid MORE (R-Texas), Rep. Leonard Lance Leonard LanceThomas Kean wins GOP primary to take on Rep. Tom Malinowski Gun debate to shape 2020 races GOP fears Trump backlash in suburbs MORE (R-N.J.) and Rep. Pete King Peter (Pete) KingTrump holds private funeral service for brother Robert Trump at White House Cheney clashes with Trump Coronavirus Report: The Hill's Steve Clemons interviews Rep. Sean Patrick Maloney MORE (R-N.Y.).

So far, all have indicated they are running for another term.

When asked if he would retire after 12 terms in the House, Frelinghuysen replied tersely, “Certainly not.”

Sessions, too, said he’s not going anywhere, even though Clinton beat Trump in his Dallas-area district by roughly 2 percentage points.

“I still have a good bit of things that I intend not only to get done, but to see through,” Sessions, who’s served since 2003, told The Hill. “This is an important time for our conference to express what we’re doing for the American people [and] to go help sell that fight.”

Melanie Zanona and Cristina Marcos contributed.