The Senate voted Saturday to allow witnesses in former President Trump's second impeachment trial, a stunning development that could extend the proceedings.

House Democrats prosecuting the case called for a single immediate witness to testify: Rep. Jaime Herrera Beutler (R-Wash.), who has hammered Trump for his actions after the Capitol attack on Jan. 6.

The move came after the nine impeachment managers had spent the previous two days meticulously detailing their allegations against Trump and appeared ready to rest the case, setting up a potential Saturday vote on Trump's fate in the Senate.

But news reports emerging Friday night raised new questions about precisely when Trump knew the Capitol breach had occurred and whether he knew that his own vice president, Mike Pence, was being targeted by the mob.

"Needless to say, this is an additional, critical piece of corroborating evidence further confirming the charges before you, as well as the president's willful dereliction of duty," lead impeachment manager Rep. Jamie Raskin (D-Md.) said. "For that reason, and because this is the proper time to do so under the [Senate impeachment rules], we would like the opportunity to subpoena congresswoman Herrera [Beutler] regarding her communications with [House Minority Leader Kevin McCarthy (R-Calif.)] and to subpoena her contemporaneous notes."

Raskin said the deposition could happen remotely by Zoom and that Democrats would request "an hour or less" of her time.

The idea was immediately rejected by Michael van der Veen, one of Trump's defense attorneys, who said he'd need more than 100 new witnesses to thoroughly investigate the circumstances in question thoroughly.

He called on senators to reject the call for new evidence, accusing Democrats of trampling on Trump's right to due process. The single article of impeachment, he added, is for "incitement of insurrection" and therefore the Senate jurors should ignore anything Trump did after the attack as immaterial to the case. 

"It's about the incitement. It's not about what happened afterwards," he said.

It's unclear how the process will play out next. The Senate will have to adopt a set of rules governing any new testimony, which are likely to include caps on the number witnesses and time restrictions on the testimony itself. Senators from both parties were huddling Saturday morning to decide next steps. 

"The attempt at the moment is to see what the parameters of witnesses might be," said Sen. Jerry Moran (R-Kan.). "But it's got to be fair and not lopsided."

Moran's entreaty for witness parity will likely be welcomed by Democrats, who have called for just one witness and are eager to wrap up the trial and move on to President Biden's legislative agenda, including a massive new round of coronavirus relief. 

Herrera Beutler, one of 10 Republicans to vote for Trump's impeachment last month, has surfaced as a key figure in that saga. She said she was briefed on a call between Trump and McCarthy, who had urged the president to defuse the attack with public statements calling off the rioters.

Trump had rejected McCarthy's entreaty, according to Beutler's account, which she amplified in a statement Friday night, calling on more Republicans to speak out against the president's conduct that day.

"When McCarthy finally reached the president on January 6 and asked him to publicly and forcefully call off the riot, the president initially repeated the falsehood that it was antifa that had breached the Capitol," Herrera Beutler said in her statement.

"McCarthy refuted that and told the president that these were Trump supporters. That’s when, according to McCarthy, the president said: 'Well, Kevin, I guess these people are more upset about the election than you are,'" the statement added.

Updated: 11:24 a.m.