WASHINGTON — President Donald Trump’s defense lawyers said their opening foray this weekend in the impeachment trial was only a “sneak preview” to Monday’s main event — but that was before revelations that Trump told John Bolton he wouldn’t lift a hold on military aid to Ukraine until it investigated his political rivals, according to the president’s former national security adviser.

That exchange, described in a draft of Bolton’s book and first reported by The New York Times, severely undercuts the defense that Trump’s counsel sought to showcase this week, and bolsters Democrats’ calls for witnesses in the trial. The Los Angeles Times could not immediately independently confirm the account.

“There can be no doubt now that Mr. Bolton directly contradicts the heart of the President’s defense and therefore must be called as a witness,” the House managers responded Sunday night in a statement. “During our impeachment inquiry, the President blocked our request for Mr. Bolton’s testimony. Now we see why.”

Bolton said he wished to testify as part of the impeachment proceedings against Trump, but the White House — which has a draft of the book, according to The New York Times — issued a blanket order blocking officials from providing testimony and documents, broadly citing executive privilege.

A partial roster of Trump’s legal team began its defense of the president on Saturday, arguing over two hours that he “did nothing wrong” when he pushed Ukraine to open investigations into the Bidens as he withheld a White House meeting and security assistance.

Trump’s team reserved its more incendiary attacks for Monday, when they planned to try to shift scrutiny onto former Vice President Joe Biden and his son Hunter — effectively giving Trump the public inquest against his potential rival in the 2020 presidential election that he sought from Ukraine.

Kenneth W. Starr, the prosecutor whose investigation led to President Bill Clinton’s impeachment, and Alan Dershowitz, the veteran defense attorney, have yet to appear in the Senate chamber as part of Trump’s defense team. But they are expected to take the lead for the bulk of the counteroffensive this week, which could last just one day.

Last week, seven House managers argued that Trump abused the power of the presidency and then obstructed Congress to cover it up, filling the 24 hours allowed by Senate rules with three days’ worth of PowerPoint presentations, video clips, detailed timelines and impassioned speeches. They called for Republican colleagues to support subpoenas for witnesses and documentation, hoping to get at least four to join them for key votes this week.

Dershowitz said the Democrats were overcompensating for a dearth of evidence with political persuasion.

“Much of what was presented by the Democrats were not impeachable offenses,” Dershowitz said Sunday on Fox News. “They were campaign ads.”

Trump’s counsel made clear they wouldn’t use their full 24 hours to make the case that Trump should be acquitted, an almost certain outcome in the Republican-controlled Senate.

Trump, the most important audience for the Republican Party, as well as his lawyers, trumpeted his approval on Sunday for the abrupt, aggressive defense strategy.

“The Impeachment Hoax is a massive election interference the likes of which has never been seen before,” Trump said in one of a number of tweets Sunday morning about the trial. “In just two hours the Radical Left, Do Nothing Democrats have seen their phony case absolutely shredded.”

With Trump complaining that the Saturday session was “Death Valley in T.V.,” Sen. Roy Blunt, R-Mo., predicted his team would go big on Monday. “They might have some sense that they need a little prime time themselves,” Blunt said.

As the chances of Republican support for witnesses appeared to dim, Senate Majority Leader Mitch McConnell, R-Ky., was strongly considering moving quickly to an acquittal vote, potentially as soon as Friday. Per the trial’s rules, if efforts to subpoena additional witnesses or evidence fail this week, Republicans could effectively shut down debate and quickly call for the up-or-down vote to determine whether Trump will remain in office.

But those preliminary plans may have been scrambled by the Bolton news.

Trump continued his lawyers’ line of argument on Sunday that the impeachment was merely a political vendetta, singling out Rep. Adam Schiff, D-Calif., the lead House manager and Intelligence Committee chairman, in a thinly veiled threat.

“He has not paid the price, yet, for what he has done to our Country!” Trump tweeted.

Schiff not only spoke far more than the other managers over their three days of trial arguments on the floor last week — but he also spoke more than did 77 senators in all of 2019, according to data from C-Span, underscoring how little debate there was in the Senate at all.

The California congressman pointed to the president’s remarks on Sunday as further evidence of the difficulty for Republican senators of standing up to the president.

“This is a wrathful and vindictive president; I don’t think there’s any doubt about it,” Schiff said on NBC’s “Meet the Press.” “If we can’t even get the senators to agree to call witnesses in a trial, it shows you just how difficult that moral courage is.”

Trump’s defense counsel and some Republican allies in the Senate have spent days priming the president and the public for the trial to enter a more provocative stage, seeking to keep the Bidens front and center.

Sen. Lindsey Graham, R-S.C., repeatedly claimed that “no one” had looked into the Bidens’ activities in Ukraine. But he also said he’d vote against witnesses, whether the Bidens or Bolton.

“If we begin to open this up, the court of impeachment, to the Hunter Biden, Joe Biden thing, where does that take us?” Graham said. “How long does that take? What acrimony does that create for the country?

“You’re asking me to violate executive privilege with John Bolton — I’m not going to do it,” he continued. “But I’m also not going to use the 53 votes we have to selectively call one set of witnesses. I think the best thing for the Senate is to judge the case that has been made.”

Hunter Biden joined the board of Burisma, a Ukrainian energy company, in April 2014. At that time, Ukraine and its fight against corruption was in his father’s vice presidential portfolio.

In the nearly six years since — roughly half of which Republicans controlled both chambers of Congress — no evidence of criminal wrongdoing by the Bidens has emerged.

“Donald Trump is so terrified of facing Joe Biden that he became the only president in American history to attempt to coerce a foreign nation into lying about a political rival,” Andrew Bates, the rapid response director for Joe Biden’s 2020 campaign, told the Los Angeles Times in a statement Sunday.

Jay Sekulow, Trump’s private counsel, said Saturday in the Senate that Trump was right not to “blindly” follow the U.S. intelligence community’s assessments that it was Russia, not Ukraine, that meddled in 2016 on his behalf.

“They kept telling you it was Russia alone that interfered in the 2016 election,” Sekulow said, “but there is evidence that Ukraine also interfered.”

In November, U.S. intelligence officials told senators and their aides that this debunked theory had been pushed by Russia for years as part of a long-running effort to frame Ukraine for Russia’s own interference. The officials warned that Moscow was intensifying its misinformation campaign as the 2020 election approached.

Schiff argued Sunday that the attempt by Trump’s counsel to justify his distrust of the U.S. intelligence community only underscored the continued danger if he remained in office.

“He continues to believe Russian propaganda over his own intelligence agencies, over his own FBI director,” Schiff said on “Meet the Press.” “And that makes him dangerous to our country.”

The Bidens, Schiff said, are being used as a political bogeyman to distract from the facts of the case against the president.

“Hunter Biden,” he said, “can’t tell us anything.”

(Times staff writer Jennifer Haberkorn contributed to this report.)