In the summer of 1950, outraged by Joseph McCarthy’s anti-Communist inquisition, Margaret Chase Smith, a Republican senator from Maine, stood to warn her party that its own behavior was threatening the integrity of the American republic. “I don’t want to see the Republican Party ride to political victory on the four horsemen of calumny — fear, ignorance, bigotry and smear,” she said. “I doubt if the Republican Party could — simply because I don’t believe the American people will uphold any political party that puts political exploitation above national interest. Surely, we Republicans aren’t that desperate for victory.”

Senator Smith surely knew her “Declaration of Conscience” would not carry the day. Her appeal to the better angels of her party was not made in the expectation of an immediate change; sometimes the point is just to get people to look up. In the end, four more years passed before the bulk of the Republican Party looked up and turned on Senator McCarthy — four years of public show trials and thought policing that pushed the country so hard to the right that the effects lasted decades. The problem with politicians who abuse power isn’t that they don’t get results. It’s that the results come at a high cost to the Republic — and to the reputations of those who lack the courage or wisdom to resist.

The Republican Party is again confronting a crisis of conscience, one that has been gathering force ever since Donald Trump captured the party’s nomination in 2016. Afraid of his political influence, and delighted with his largely conservative agenda, party leaders have compromised again and again, swallowing their criticisms and tacitly if not openly endorsing presidential behavior they would have excoriated in a Democrat. Compromise by compromise, Donald Trump has hammered away at what Republicans once saw as foundational virtues: decency, honesty, responsibility. He has asked them to substitute loyalty to him for their patriotism itself.

Mr. Trump privately pressed Ukraine to serve his political interests by investigating a political rival, former Vice President Joe Biden, as well as by looking into a long-debunked conspiracy theory about Democratic National Committee emails that were stolen by the Russians. Mr. Trump publicly made a similar request of China. His chief of staff, Mick Mulvaney, said publicly on Thursday that the administration threatened to withhold military aid from Ukraine if it did not help “find” the D.N.C. servers .