A potential blue wave and string of Republican retirements could wipe out a huge chunk of GOP moderates this fall, leaving an even more conservative House caucus behind them.

Some of the most competitive races this election cycle feature centrist Republicans like Mike Coffman Michael (Mike) Howard CoffmanColorado mayor says he called protesters 'domestic terrorists' out of 'frustration' Colorado governor directs officials to reexamine death of Elijah McClain in police custody Petition demanding justice for Elijah McClain surpasses 2 million signatures MORE of Colorado, 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 of New Jersey and Brian Fitzpatrick Brian K. FitzpatrickHopes for DC, Puerto Rico statehood rise Florida Democrat introduces bill to recognize Puerto Rico statehood referendum DCCC reserves new ad buys in competitive districts, adds new members to 'Red to Blue' program MORE of Pennsylvania.

And the GOP’s most prominent and outspoken moderate voices — including Reps. Ryan Costello Ryan Anthony CostellloTrump struggles to stay on script, frustrating GOP again Bottom line Former GOP Rep. Costello launches lobbying shop MORE (Pa.) and Ileana Ros-Lehtinen Ileana Carmen Ros-LehtinenEx-Florida GOP congresswoman under federal investigation: report 'Trump show' convention sparks little interest on K Street Shalala to face Salazar in Florida rematch MORE (Fla.) and former Rep. 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 (Pa.) — have decided to head for the exits instead of face tough reelection battles back home.

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Centrist GOP lawmakers are in a difficult political climate in the era of Trump, where they are constantly walking a political tightrope as they try to respond to a never-ending stream of White House controversies without upsetting their Republican base.

“Look, it’s a nasty political environment and moderate Republicans in competitive seats get the worst of it,” Costello told The Hill. “The atmospherics are against Republicans. But there are many battle-tested Republicans that possess the acumen to pull through.”

That tough dynamic was on full display Friday, when Lance joined a handful of moderate Republican senators in calling for an FBI investigation into sexual assault allegations made against Trump’s Supreme Court nominee, Brett Kavanaugh.

Trump agreed to authorize the FBI to conduct a supplemental background investigation into Kavanaugh. The decision comes a day after Kavanaugh and one of his accusers, Christine Blasey Ford, testified publicly before the Judiciary panel in a rollercoaster hearing about the sexual assault allegations.

“Dr. Ford’s testimony yesterday was credible and believable. But I also found Judge Kavanaugh to be credible in his own defense,” Lance said in a statement. “It’s in the best interest of the nation that a full and thorough examination by federal law enforcement of these serious charges against Judge Kavanaugh occurs.”

Lance represents just one of the dozens of vulnerable House seats that Republicans are defending this election cycle, where Democrats who are furious with Trump and energized by the “Me Too” movement hope to win back the House and Senate majorities. Independent and moderate Republican women in the suburbs are expected to form a crucial voting bloc that could determine the outcome of the November elections.

Other Republican centrists facing tough reelection races include Reps. Carlos Curbelo Carlos Luis CurbeloGOP wants more vision, policy from Trump at convention Mucarsel-Powell, Giménez to battle for Florida swing district The Memo: GOP cringes at new Trump race controversy MORE (Fla.), Will Hurd William Ballard HurdHillicon Valley: Oracle confirms deal with TikTok to be 'trusted technology provider' | QAnon spreads across globe, shadowing COVID-19 | VA hit by data breach impacting 46,000 veterans House approves bill to secure internet-connected federal devices against cyber threats House Democrats' campaign arm reserves .6M in ads in competitive districts MORE (Texas) and Jeff Denham Jeffrey (Jeff) John DenhamBottom line Bottom line Lobbying world MORE (Calif.). The trio led an unsuccessful, insurgent rebellion this summer to circumvent GOP leadership and force floor action on immigration legislation using a so-called discharge petition.

Rep. John Katko John Michael KatkoThis week: House returns for pre-election sprint Hillicon Valley: Simulated cyberattack success | New bill for election security funding | Amazon could be liable for defective products Lawmakers introduce bill to help election officials address cyber vulnerabilities MORE (R-N.Y.), co-chairman of the moderate Tuesday Group who was also involved in the immigration talks, is facing a competitive reelection race, while Rep. Tom MacArthur Thomas (Tom) Charles MacArthurRepublican David Richter wins NJ primary in race to challenge Rep. Andy Kim What to watch in New Jersey's primaries on Tuesday Republicans plot comeback in New Jersey MORE (R-N.J.), a former leader of the group, is locked in a tight race rated as a “toss-up” by election prognosticators.

And Reps. 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.) and Rodney Davis Rodney Lee DavisHouse passes legislation to boost election security research House Republicans investigating California secretary of state's contract with Biden-linked firm House Democrats' campaign arm releases ads hitting 10 Republicans on health care MORE (R-Ill.), two centrists from the Midwest, could also be in trouble if a blue wave materializes this fall.

It’s not unusual for either party to see their moderates in danger, especially since swing districts are usually represented by lawmakers closer to the middle.

And to be sure, there are some ultra conservative members who are also deeply at risk of losing their seats, including House Freedom Caucus members Rod Blum (R-Iowa) and Dave Brat (R-Va.)

But the majority of the GOP’s vulnerable seats — and a large share of the conference’s retiring members — are more closely aligned to the center.

“I think most would agree with me when I say that for every seat we lose in the House, the center of gravity in the conference moves right,” Costello said.

Centrist lawmakers, however, are used to running in competitive races every election cycle, which is giving some hope to the GOP.

Some of those moderate Republicans are hoping to sell their center-right principles and calls for sensible bipartisanship on the campaign trail, where they say their constituents have grown fed up with the extreme polarization in Congress.

A number of vulnerable lawmakers signed a pledge this week promising to withhold their votes for the next Speaker unless the candidate agrees to dramatically reform the House rules.

The ambitious effort to “break the gridlock” is designed to make the House more inclusive and bipartisan, in part by making it easier for members to offer and pass their own legislation, especially if it has support from both parties.

“Of all the issues that concern residents in the district that I serve, the most important issue is not health care or taxation or immigration,” Lance said at a press conference earlier this week. “The most important issue is bipartisan cooperation. ... And that is what we are attempting to do.”

— Scott Wong contributed