The Senate chaplain on Friday warned senators "we reap what we sow" ahead of a vote on whether to call witnesses in President Donald Trump's impeachment trial.

"Lord help them to remember, that they can't ignore you and get away with it. For we always reap what we sow," Chaplain Barry Black said.

Senators were expected to hold a vote on witnesses later in the day, with Republicans poised to strike down the motion.

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The Senate chaplain, Barry Black, offered an ominous warning to senators in the opening prayer of President Donald Trump's impeachment trial on Friday, which came ahead of an expected vote on whether to call witnesses.

"Eternal Lord God, you have summarized ethical behavior in a single sentence: Do for others what you would like them to do for you," Black said.

The chaplain added, "Remind our senators that they alone are accountable to you for their conduct. Lord help them to remember, that they can't ignore you and get away with it. For we always reap what we sow."

Senators were expected to vote on witnesses later on Friday, with Republicans poised to stand against the motion and strike it down.

Democrats have been pushing hard for witnesses, emphasizing that not calling any would be a break from historical precedent. All 15 Senate impeachment trials prior to Trump's saw witnesses called, including those for the only other US presidents who were impeached.

"We agree with the president's council on this much, this will set a new precedent. This will be cited in impeachment trials from this point to the end of history," Democratic Rep. Adam Schiff of California, the lead House impeachment manager, said on Friday while making the case for witnesses.

Democrats want to hear from Trump's former national security adviser, John Bolton, in particular. In a forthcoming book that's been reported on by the New York Times over the past week, Bolton undermined one of Trump's key defenses against impeachment, pointing to an explicit quid pro quo that linked roughly $400 million in military aid to Ukraine to investigations into the president's political rivals.

Bolton also reportedly wrote that Trump asked him to get involved in the efforts to pressure Ukraine in a meeting more than two months before the president's July 25 phone call with his Ukrainian counterpart that's been at the center of the impeachment proceedings.

The former national security adviser said Trump's personal lawyer, Rudy Giuliani, the White House counsel, Pat Cipollone, and the acting White House chief of staff, Mick Mulvaney, were also in attendance. Trump and Giuliani denied the meeting took place.

Cipollone has served as Trump's lead attorney in the impeachment trial, and House impeachment managers have called for him to disclose any firsthand evidence in light of the Times' reporting.