Republicans have had their differences these past few years. Most have supported President Trump; a few have not. Some of the president’s supporters have been enthusiastic; many have not. Some of the reluctant Trump supporters have expressed reservations at certain times; many have not.

But with the revelations of the last week, and the launch of a formal impeachment inquiry, we are at a new moment. This is obviously the case for Republicans in Congress, who will have to vote on impeachment and perhaps on conviction. They have a unique part to play in this drama; the rest of us are merely observers or advisers. All we can really now ask of members of Congress is to keep an open mind and to evaluate the facts as they emerge.

But we already have learned enough to know that the government whistle-blower is correct to say “that the President of the United States is using the power of his office to solicit interference from a foreign country in the 2020 U.S. election.” We know this latest instance is part of a history of repeated injuries and usurpations. We may not yet know whether removal from the office to which President Trump was elected is warranted. But surely we know enough to judge that Mr. Trump does not deserve renomination for that office for an additional four years.

The Republican Party faces a binary choice. It either will or will not renominate Donald Trump in 2020. (And if President Trump is removed as a consequence of impeachment and conviction, or if he resigns, the G.O.P. either will or will not nominate as its standard-bearer a newly sworn-in Mike Pence, who will have been at Mr. Trump’s side for his entire administration and has been a fervent defender of the president.)