Yet in announcing his retirement, Mr. Walden did not sound confident about his party’s chances in the 2020 contests — “a path exists for Republicans to recapture a majority in the House,” he said — and his decision to leave Congress underscored how reluctant many Republicans have grown, as the impeachment of President Trump gallops ahead, to sign up for another two years in the minority.

Weary of spending another Congress with little power in the majority-dominated House and the drama of having to answer daily for the president’s provocative comments and actions, 20 other House Republicans have announced they would not seek re-election in 2020.

Representative Tom Emmer of Minnesota, the chairman of House Republicans’ campaign arm, called Mr. Walden a “class act and political juggernaut who will be sorely missed.” In a statement, Mr. Emmer also said that Mr. Walden had pledged to remain “heavily engaged” in ensuring his seat, a conservative stronghold in the center of the state, would stay in Republican control.

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Mr. Walden is now the fourth lawmaker who serves as the top Republican on a prominent House committee to announce his retirement this year. The others are Representatives Mac Thornberry of Texas, the senior Republican on the Armed Services Committee; Rob Bishop of Utah on the Natural Resources Committee; and K. Michael Conaway of Texas on the Agriculture Committee.

If House Republicans recaptured the chamber in 2020, Mr. Walden could regain the chairmanship of the Energy and Commerce committee and hold it until 2022, according to House rules.