Consider the congressional math. Democrats hold a 235-197 majority (with one independent). Two seats are vacant, so 217 votes are needed to impeach. Thirty-one Democrats hold districts Mr. Trump won in 2016. To get to 217 without Republican help, at least 12 of those 31 would have to vote for impeachment. The pressure to do so will be immense, making it a defining vote. At a trial, Senate moderates would also have to take a stand.

Those who oppose impeachment would risk a primary challenge from the left. Those who support it would endanger themselves in the general election. Either way, the left wins by converting or purging moderates. Controlling both the nominee and the Democratic caucuses in Congress would strengthen the left’s hand considerably, whether or not it was good for the party as a whole. Rules, funding, personnel—all would bend to the left’s liking.