When William S. Cohen, a young Republican House member from Maine, broke with his party over Watergate and joined Democrats in demanding the Nixon White House tapes in 1973, he figured his fledgling political career was dead.

“I had come to the conclusion based on all of the hate mail I had gotten, all of the flak I had gotten, that I wasn’t coming back,” recalled Mr. Cohen, who had been elected less than a year earlier. “I was O.K. with that.”

Mr. Cohen, of course, did come back — and he went on to a distinguished career in the House, three terms in the Senate and a stint in the Clinton administration as secretary of defense after disclosures contained in the tapes led to President Richard M. Nixon’s resignation.

As he watches events unfold today in Washington, Mr. Cohen believes congressional Republicans are wrongly putting party first by steadfastly backing President Trump’s refusal to cooperate with Congress and trying to shut down further investigation into whether the president obstructed justice.

“He is basically thumbing his nose at the Congress itself, saying I don’t recognize you,” Mr. Cohen, who heads an international consulting and lobbying firm, said in an interview. “I am surprised there aren’t more defenders of the Constitution. They are there to be a check on abuse of power. If they are willing to submit that to the executive, then they have no business being in office.”

Republican opposition in both the House and the Senate to pushing ahead with such inquiries is crucial because, as history has shown with the Watergate and Iran-contra scandals, such investigations can gain legitimacy only when members of the president’s own party support them. Intense partisanship surrounded those investigations as well, but with Republican lawmakers like Mr. Cohen and a handful of others showing some willingness to aggressively question the administration, the White House was forced to be more accountable and cooperative.