Update: The Senate voted to acquit President Trump on Feb. 5.

On Wednesday, the Senate will most likely vote to acquit President Trump. The vote should send shock waves through our democracy. It is not so much that the Senate is absolving him of all charges of obstruction of Congress and abuse of power; the shock waves should emanate from how his Republican allies go about it. By rejecting a conviction, the party will demonstrate that it believes anything goes in winning elections.

Most Republican senators have insisted that in pressuring Ukraine to investigate the Bidens, Mr. Trump did nothing wrong. A few say he crossed a line, but they have never explained why his actions fall short of conviction, or what it would take for them to consider a conviction.

Rather than reining in a president who clearly abused his power for personal gain, most Republicans have conceded to Mr. Trump’s overarching defense: that his re-election would serve the public interest. That argument was enough, for his Senate allies, to override campaign finance laws and the norms of governance that have prevailed in our country until this presidency.

This defense is a natural outgrowth of the unitary executive theory, a legal doctrine advanced by apologists for the imperial presidency, including Attorney General William Barr. It was Alan Dershowitz, the Harvard law professor representing Mr. Trump, who gave this idea its most outrageous frame: “Every public official that I know believes that his election is in the public interest,” he said. “And if a president does something which he believes will help him get elected in the public interest, that cannot be the kind of quid pro quo that results in impeachment.”