Opinion: Surprise, Mr. President. John Bolton Has the Goods.

Who’s telling the truth about Ukraine? There’s one way to find out.

It’s just possible that common sense and reality have a shot at prying open the doors to the Senate chamber after all. After Republican senators claimed that it was perfectly reasonable to put a United States president on trial without hearing from any witnesses, a few of them are showing signs of recognizing that the truth matters. Or, at least, that the American people believe it does.

What’s changed? Shocking but not surprising revelations from John Bolton’s book manuscript, which The New York Times reported over the weekend, have made impossible to ignore what everyone has known for months: President Trump withheld hundreds of millions of dollars in military aid to Ukraine to benefit himself politically, and against the strenuous objections of his top aides and both parties in Congress.

On Monday morning, Mitt Romney, of Utah, said, “I think it’s increasingly likely that other Republicans will join those of us who think we should hear from John Bolton.”

It’s refreshing to hear those words. And yet the fact that such a statement is noteworthy at all tells you how far from responsible governance Republicans have strayed. They hold 53 seats in the Senate, and yet the nation is waiting on just four — four!— to do the right thing and agree to call Mr. Bolton, the former national security adviser, and other key witnesses to testify in Mr. Trump’s impeachment trial.

A far more representative attitude in the Republican caucus was expressed by Roy Blunt, of Missouri, who said on Monday, “Unless there’s a witness that’s going to change the outcome, I can’t imagine why we’d want to stretch this out for weeks and months.” With this tautology Senator Blunt gives away the game: All witness testimony to date — all presented as part of the House impeachment proceedings — has only strengthened the case against Mr. Trump, but Republicans will not vote to convict him under any circumstances. By definition, then, no witness in the Senate could possibly change the outcome.The reporting on Mr. Bolton’s manuscript, which is scheduled for publication in March, has scrambled that strategy. Mr. Bolton’s foreign-policy disagreements with Mr. Trump have been public knowledge for months. Last fall, Fiona Hill, a Russia expert and former Bolton aide, testified in the House that Mr. Bolton was alarmed by Mr. Trump’s aid-for-investigations scheme, which Mr. Bolton characterized as a “drug deal.”In the manuscript, detailed descriptions of which were leaked to The Times, he recounts nearly a dozen instances in which he and other top administration officials pleaded with Mr. Trump to release the aid, to no avail. He describes Mr. Trump’s fixation on conspiracy theories about Ukrainian interference in the 2016 election, and about the supposed corruption of Marie Yovanovitch, the American ambassador to Ukraine. He says that Secretary of State Mike Pompeo admitted privately to him that he knew there was nothing to the theories regarding Ms. Yovanovitch, whom Mr. Trump fired last spring.

Mr. Bolton, a hard-line conservative with decades of service in Republican administrations, is no anti-Trump zealot, which makes his allegations against the president that much more devastating. And his decision to tell these stories publicly nearly certainly waives any claims of executive privilege Mr. Trump might try to assert over their communications.

Let’s not forget the newly revealed evidence that came to light on Saturday, in the form of a tape recording released by the lawyer for Lev Parnas, who had worked for Rudy Giuliani, the president’s personal lawyer, in the Ukraine scheme. Mr. Trump has denied even knowing Mr. Parnas, but on the tape the two men can be heard in conversation at a dinner in April 2018. “Get rid of her,” Mr. Trump said of Ms. Yovanovitch. “Get her out tomorrow. Take her out. O.K.? Do it.”

In a late-night tweet, Mr. Trump angrily denied Mr. Bolton’s allegations. “I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens,” Mr. Trump wrote.

You know what would be a good way to figure out who’s telling the truth? Subpoena Mr. Bolton to testify under oath.

This isn’t a close call. A majority of Americans of all political stripes want to hear from Mr. Bolton, at the least. They believe, as do congressional Democrats, that you can’t vote on whether to remove a president from office without getting the fullest possible account of his alleged offenses.

But Senate Republicans have so far refused to hear from any witnesses or to demand any documents, following the lead of Mitch McConnell, the majority leader, who has never hesitated to undermine the country’s institutions if he thinks doing so will help his party. Mr. McConnell and nearly all in his caucus seem to imagine that if they block their eyes and ears and let their mouths run, the turbulence of impeachment will eventually pass.

This is a risky strategy. One reason good lawyers insist on deposing witnesses and subpoenaing documentary evidence is to avoid any unwelcome surprises at trial. Mr. Bolton has now provided the latest of those surprises. It is surely not the last. The most galling part is that Republicans have already admitted how bad the president’s behavior was. Back in September, Senator Lindsey Graham, the South Carolina Republican and one of Mr. Trump’s staunchest defenders, said: “What would’ve been wrong is if the president had suggested to the Ukrainian government that if you don’t do what I want you to do regarding the Bidens, we’re not going to give you the aid. That was the accusation; that did not remotely happen.”

Except that it did, as Mr. Bolton is apparently willing to say under oath. Republicans don’t want him to do that because they don’t want Americans to exercise the simple good judgment that Mr. Graham once did.