WASHINGTON — President Trump’s impeachment trial enters a pivotal week as his defense team resumes its case and senators face a critical vote on whether to hear witnesses or proceed directly to a vote that is widely expected to end in his acquittal. The articles of impeachment charge Trump with abuse of power and obstruction of Congress.

The debate over witnesses received a jolt Sunday night when The New York Times reported that Trump told his national security adviser he wanted to maintain a freeze on military assistance to Ukraine until it launched political investigations into his Democratic rivals. The newspaper said John Bolton’s description of his exchange with Trump appears in drafts of his forthcoming book.

Not only did the revelation challenge the defense offered up by Trump and his attorneys, it sparked a fresh wave of demands by Democrats that Senate Republicans allow Bolton and others to testify at the trial.

The Capitol Hill maneuvering will be complemented by high-stakes efforts on both sides of the aisle to claim political advantage from the proceedings as the presidential nominating season kicks off in Iowa on Feb. 3.

What to watch as the Senate impeachment trial resumes at 1 p.m. Monday.

STAR TURN IN DEFENSE

After a two-hour opening argument Saturday, Trump’s defense team will lay out its case in depth beginning Monday. White House counsel Pat Cipollone said Trump’s lawyers don’t expect to take the full 24 hours allotted to them, but there will be arguments from some familiar faces.

Harvard law professor emeritus Alan Dershowitz, former independent counsel Ken Starr and former Florida Attorney General Pam Bondi will speak on specific topics.

Dershowitz said Sunday he would argue that the charges against Trump are too minor to warrant the president’s removal from office under the Constitution. “Even if true, they did not allege impeachable offenses,” Dershowitz told “Fox News Sunday.”

The Trump team has also teased the notion that it would draw attention to Joe Biden and his son Hunter, who served on the board of a Ukraine gas company Burisma, while the elder Biden was vice president. An extended focus on Joe Biden, one of the leading Democratic presidential contenders, could mean blowback from even some of the GOP members of the Senate.

President Trump’s personal attorney Jay Sekulow, center, stands with his son, Jordan Sekulow, left, and White House Counsel Pat Cipollone, while arriving at the Capitol in Washington Saturday during the impeachment trial of President Trump on charges of abuse of power and obstruction of Congress. Julio Cortez/Associated Press

QUESTION TIME

Once Trump’s team concludes, senators will have 16 hours to ask questions of both the House impeachment prosecutors and the president’s legal team. Their questions must be in writing.

Chief Justice John Roberts will read the questions aloud. He is expected to alternate between both sides of the aisle. Many senators have been talking copious notes throughout the trial in preparation for the question-and-answer time.

Sen. John Barrasso, R-Wyo., told reporters Saturday that Republicans expected to get together on Monday to start formulating a list of questions. “We will meet as a conference and decide what questions we want to pose, what the order may be of those of those questions,” he said.

WILL THEY OR WON’T THEY

Under the Senate rules passed last week, senators will get another chance to vote whether to consider new witnesses and evidence after the Q&A time is elapsed. Four Republicans would have to break ranks to join Democrats in the GOP-controlled Senate to extend the trial for an undetermined amount of time. If that happens, expect a bitter fight over which witnesses might be called and which documents might be subpoenaed.

After the New York Times report went online Sunday night, Democrats quickly sought to ramp up the pressure on their Republican counterparts.

John Bolton Carolyn Kaster/Associated Press

“John Bolton has the evidence,” tweeted Senate Minority Leader Chuck Schumer. “It’s up to four Senate Republicans to ensure that John Bolton, Mick Mulvaney, and the others with direct knowledge of President Trump’s actions testify in the Senate trial.”

Senate Majority Leader Mitch McConnell had no immediate comment, according to his office.

Besides Bolton, Democrats have called for testimony from Trump’s acting chief of staff, Mick Mulvaney. An attempt to call either probably would lead to a showdown with the White House, which claims both men have “absolute immunity” from being called to testify before the Senate, even in an impeachment trial. Bolton has said he would appear.

While Republicans have hoped for a speedy trial, Trump has called for the testimony of the Bidens and the intelligence community whistleblower whose summer complaint about Trump’s July telephone call with Ukraine’s leader instigated the impeachment inquiry.

But some Republicans, including Sen. Lindsey Graham, R-S.C., have expressed resistance to calling those witnesses.

If the vote fails, the Senate could move swiftly to its vote on whether to remove or acquit Trump, giving the president the result he’s been looking for as soon as the end of the week.

Senate rules also call for four hours of deliberations before voting. Since senators are required to sit silently during the trial, expect a closed session where they can deliberate in private.

A NEW TAPE

Trump’s lawyers argued Saturday that no one knows what Trump’s motives were on holding up military assistance to Ukraine. A recording obtained by The Associated Press hours later suggests the president well understood that assistance was a point of leverage over Ukraine.

The recording is of 2018 meeting at the Trump Hotel in Washington that Trump had with donors. including two now-indicted associates of his personal attorney, Rudy Giuliani. The audio portion includes Trump inquiring about Ukraine, “How long would they last in a fight with Russia?” He later calls for the firing of the U.S. ambassador to Ukraine, Marie Yovanovitch.

The recording contradicts the president’s statements that he didn’t know the Giuliani associations, Lev Parnas and Igor Fruman. They are key figures in the investigation who were indicted last year on campaign finance charges.

If new evidence and witnesses are allowed, the recording could take center stage in the Senate proceedings.

THE POLITICS

The trial is resuming with one week to go until the Iowa caucuses, and is again keeping four Democratic contenders – Sens. Bernie Sanders, Elizabeth Warren, Michael Bennet and Amy Klobuchar – in Washington instead of campaigning at a critical point in the race.

While they are trapped in Washington, Trump will venture outside the capital as he seeks both to exert political retribution on Democrats who impeached him and reward a party-switching lawmaker who backed him in the House.

Trump will hold a rally Tuesday in New Jersey to repay the favor to Rep. Jeff Van Drew, who became a Republican last month after voting against the articles of impeachment as a Democrat. And Trump is set to appear in Iowa on Thursday, days before the caucuses.

Meanwhile, Trump is already looking ahead to his likely acquittal, whenever it may come, promising that Democrats will face consequences for trying to remove him from office. “Shifty Adam Schiff is a CORRUPT POLITICIAN, and probably a very sick man,” Trump tweeted Sunday. “He has not paid the price, yet, for what he has done to our Country!”

Schiff, D-Calif., is chairman of the House Intelligence Committee and the lead impeachment manager. Asked on NBC’s “Meet the Press” whether he viewed the tweet as a threat, Schiff replied, “I think it’s intended to be.”

NADLER ABSENT

Rep. Jerrold Nadler, one of the House managers for the Senate trial, will miss Monday’s proceedings because of his wife’s illness.

In a statement, the New York Democrat said he would be in New York that day to discuss with doctors his wife’s ongoing treatment for pancreatic cancer. As chairman of the House Judiciary Committee, Nadler has been a key member of the Democratic team investigating and prosecuting the case against the president.

Associated Press writers Lisa Mascaro, Eric Tucker, Laurie Kellman and Mary Clare Jalonick contributed to this report.