WASHINGTON — An expected verdict in Donald Trump’s impeachment trial was thrown into doubt Saturday after the Senate voted to allow witnesses to be called, a result of a surprise push by Democrats to summon a Republican congresswoman who has said she was told that the former president sided with the mob as rioters were attacking the Capitol.

The last-minute move to throw open the trial record came on a day when the Senate had been expected to hand Trump his second impeachment acquittal, on the charge of “incitement of insurrection,” for his alleged role in stoking the mob that attacked the Capitol on Jan. 6 while Congress was meeting to formalize his election loss.

It threatened to prolong the trial for days or longer, even as the top Senate Republican, Mitch McConnell of Kentucky, privately told his colleagues he was ready to acquit Trump, confirming that an eventual conviction was exceedingly unlikely.

The vote came after Rep. Jamie Raskin of Maryland, the lead impeachment manager, said that Democrats wanted the chance to call Rep. Jaime Herrera Beutler, R-Wash., to testify. Herrera Beutler confirmed late Friday that Rep. Kevin McCarthy, the minority leader, had told her that Trump said in a phone call during the rampage that the rioters were more upset about the election than McCarthy was.

Raskin said he wanted a short deposition held virtually with the congresswoman, who was one of 10 Republicans who voted to impeach Trump, and a subpoena for her contemporaneous notes.

“We believe we’ve proven our case,” Raskin said moments after the Senate convened in the impeachment trial session Saturday morning. But he said Herrera Beutler’s statement amounted to “an additional critical piece of corroborating evidence, further confirming the charges before you.”

Five Republicans — Sens. Susan Collins of Maine, Lindsey Graham of South Carolina, Lisa Murkowski of Alaska, Mitt Romney of Utah and Ben Sasse of Nebraska — joined Democrats in a 55-45 vote to support the call for more witnesses and evidence. (Graham, who has warned that Republicans would force calls on a number of Democrats should witnesses be voted on, initially voted against the request.)