Fourteen-term GOP Rep. Peter T. King of New York said Monday that he would not seek reelection next year, highlighting a political and institutional challenge for Republicans as they seek to halt a precipitous erosion of voter support in the suburbs as their party retreats from the political center under President Trump.

King is the fifth House member of the centrist Republican Main Street Partnership to announce retirement this year and the 20th Republican to retire overall. His decision immediately put his South Shore Long Island seat near the top of Democratic target lists at a time when suburban voters continue to trend away from Republicans.

Last week’s off-year elections saw Democrats gain ground in state and local races in key suburban battlegrounds such as Bucks County, Pa., greater Indianapolis and the Hampton Roads region of Virginia — places that reliably elected Republicans for decades until Trump’s 2016 election. Since then, college-educated women in particular have abandoned Republican candidates who have been unable to separate themselves from Trump and the hard-line conservative agenda he has pursued.

“We’re having a crisis in suburban districts,” said Sarah Chamberlain, president and chief executive of the Main Street Partnership. “We have to talk to suburban women, like myself, better. We have to be addressing their concerns and we just aren’t.”

Among the most troubling results for the GOP last week included heavy losses in the collar counties surrounding Philadelphia, indicating an uphill climb for Rep. Brian Fitzpatrick (R-Pa.), who narrowly survived the 2018 Democratic wave, and continued political tail winds for Rep. Andy Kim (D-N.J.), a freshman representing the district just across the Delaware River.

Rep. Cheri Bustos (Ill.), the Democratic Congressional Campaign Committee chairwoman, said in an interview Monday that King’s retirement indicated that Democrats remain on the march in the suburbs — crediting her party’s strong fundraising and aggressive field operations with driving the spate of retirements.

Bustos said Democrats are targeting as many as 39 districts that Republicans won in 2018 by 5 points or fewer and where demographic trends are headed in Democrats’ favor. “We are putting them on notice,” she said. “We’re going to play there, and we think we’ll have some tremendous success in 2020.”

King, 75, has been a steadfast supporter of Trump’s, defending the fellow Queens native during his final two years on the House Intelligence Committee in 2017 and 2018 amid allegations of Russian collusion. But King also has a strong bipartisan streak — routinely joining Democrats in supporting gun control and efforts to aid 9/11 responders and opposing GOP efforts to limit a popular tax deduction for state and local taxes.

In addition to the expected praise from fellow Republicans, King’s announcement prompted praise from some prominent Democrats. Senate Minority Leader Charles E. Schumer (D-N.Y.) said in a tweet Monday that King “stood head & shoulders above everyone else” and “never let others push him away from his principles.”

In a statement announcing his retirement, King said the main reason for his decision was the grueling schedule — “after 28 years of spending 4 days a week in Washington, D.C., it is time to end the weekly commute.”

King denied any political impetus for his retirement, noting that his “polling numbers are as strong as they have ever been” and that he has a seven-figure campaign account. And he showed no sign of discomfort with Trump, vowing to vote against his impeachment and to support his reelection next year.

Chris Pack, a spokesman for the National Republican Congressional Committee, predicted that King’s seat would remain in Republican hands “thanks in no small part to the insane socialist agenda of Democrats in both Washington and Albany.”

But Trump has clearly been an anchor on other suburban Republicans, and the GOP’s attempts to tie Democratic candidates to far-left elements have sputtered: After the 2018 elections, only three congressional Republicans were left representing any part of the greater New York City area: King, Rep. Christopher H. Smith (N.J.) and Rep. Lee Zeldin (N.Y.).

King won reelection in 2018 with 53 percent of the vote over Democrat Liuba Grechen Shirley — his lowest percentage since first being elected in 1992. In a statement, Shirley said she was “seriously considering” another run for Congress, and national Democratic operatives believe they already have a top-tier recruit in Jackie Gordon, an Army Reserve veteran and Babylon town councilwoman.

Besides King’s Long Island seat, Democrats have hopes of picking up House seats held by retiring GOP moderates Susan Brooks (Ind.) and Will Hurd (Tex.). The seats of other centrist retirees such as Reps. Paul Cook (Calif.), John Shimkus (Ill.) and Greg Walden (Ore.) are probably out of Democratic reach, but each could be replaced by a more conservative Republican who could be less inclined to work across the aisle.

Democrats, meanwhile, are facing a demographic crisis of their own as rural white voters flee their party for Republicans. But that trend — while troubling in many statewide races and for the presidential landscape — poses less of an existential threat to the Democratic House majority, which counts only a handful of rural-district members in its ranks.

Still, the exodus of veteran lawmakers has not been an exclusively Republican problem: Three senior Democratic appropriators with reputations for bipartisanship have announced their exits in recent months, passing up the chance to influence hundreds of billions of dollars in government spending after decades of climbing up the House ranks.

Last week, Rep. Peter J. Visclosky (Ind.) announced his retirement on the 35th anniversary of his first election to Congress — joining House Appropriations Committee Chairwoman Nita M. Lowey (N.Y.) and Rep. José E. Serrano (N.Y.), chairman of the panel’s commerce, justice and science subcommittee. Visclosky, as chairman of the Defense appropriations subcommittee, is poised to direct $690 billion of Pentagon spending for 2020 — if Republicans and Democrats can agree on an overarching spending plan, which remains in doubt.

John Wagner contributed to this report.