Pelosi addresses the idea of testimony from Lev Parnas.

Speaker Nancy Pelosi said Thursday that Lev Parnas, the Soviet-born businessman who says President Trump was fully aware of efforts to dig up damaging information that would help him in the 2020 election, would be “a credible witness” during the impeachment trial, though she stopped short of saying he should testify.

“He’d be a credible witness if what he’s testifying to relates to the issue at hand — the president’s behavior,” Ms. Pelosi said of Mr. Parnas, who is under federal indictment. But she said she would leave it to the House impeachment managers to determine whether they want to seek his testimony.

The first sign that something unusual is happening in the Capitol on Thursday was apparent at every entrance door: Capitol Police officers are wearing their ties, mandatory attire for the impeachment trial. On ordinary days, the officers have the option of wearing ties but are also permitted to wear turtlenecks bearing a gold police insignia under their dress navy blue shirts.

The ties were just one sign that the Capitol is beefing up security in advance of the trial. Reporters, staff members and Capitol Police officers are required to wear special color-coded badges — maroon for reporters, brown for staff members, gray for police officers — in the Senate during the trial. Staff members from the House were not issued badges; they have been notified that they cannot cross the Capitol into the Senate while the trial is in session.

The Trump administration violated the law in withholding security assistance aid to Ukraine, a nonpartisan federal watchdog agency said on Thursday, weighing in on a decision by President Trump that is at the heart of the impeachment case against him.

The Government Accountability Office said the White House Office of Management and Budget withheld the nearly $400 million, which was allocated by Congress, for “a policy reason” in violation of the Impoundment Control Act. The decision was directed by the president himself, and during the House impeachment inquiry, administration officials testified that they had raised concerns about its legality to no avail.

The White House budget office rejected the report’s conclusions.

The White House press secretary, Stephanie Grisham, tried to dismiss the stream of new details emerging about President Trump’s pressure campaign on Ukraine via Lev Parnas, an associate of the president’s personal lawyer, Rudolph W. Giuliani.

“We’re not too concerned about it. Once again, we know that everything in the Senate is going to be fair,” Ms. Grisham told Fox News on Thursday. “It’s unfortunate that he’s now making a media tour out with a lot of the outlets that are against the president. I think that shows exactly what he’s doing.”

Ms. Grisham also criticized the House impeachment proceedings and said the White House expected that Mr. Trump’s formal response to the charges would prove he did nothing wrong.

Even as the trial was about to begin, lawmakers on both sides of the Capitol were again wading on Thursday through a trove of newly released text messages, voice mail messages, calendar entries and other records related to a campaign to pressure Ukraine to investigate Mr. Trump’s political rivals.

The evidence, handed over by Lev Parnas, adds significant new detail to the public record about how the pressure campaign undergirding the House impeachment charges played out and who knew about it. On Wednesday, Mr. Parnas also told The New York Times that he believed Mr. Trump knew that he and Rudolph W. Giuliani, the president’s personal lawyer, were working to dig up dirt on his political rivals.

The House managers could try to weaponize pieces of the material once the trial begins, but for now, they hoped the gusher of information would increase pressure on Senate Republicans to agree to call witnesses and collect new evidence during the trial.

History will be made swiftly on Thursday, as the Senate initiates only the third presidential impeachment trial in American history in three hours or less. The events begin at noon, and senators may be on airplanes out of town before business hours are over.

Here is a run-down of the day’s proceedings, based on guidance from Senate officials:

• Before the impeachment trial gets underway, the Senate has some big unfinished business to wrap up. Around 11 a.m. Eastern, senators are expected to vote to approve Mr. Trump’s long-sought trade agreement with Canada and Mexico.

• At noon, the newly appointed House managers will assemble on their side of the Capitol and process for a second consecutive day to the Senate chamber. This time, they will enter with the intention of exhibiting their articles and being introduced to senators. This entails reading the articles out loud, in full, as all 100 senators sit quietly at their desks listening.

• The Senate plans to recess whenever the House managers are done with their presentation. Both parties will retreat to their usual luncheons to discuss strategy for the coming trial.

• Chief Justice John G. Roberts Jr. will be escorted into the Senate chamber around 2 p.m. to preside over the trial. Senator Charles E. Grassley of Iowa, the Senate president pro tempore, will swear the chief justice in. His first task is to administer an oath to all senators, who must swear to do “impartial justice.” Senators will respond orally, but they must each sign an oath book to sit in trial.

• At some point thereafter, the Senate will issue a summons to Mr. Trump informing him of its trial and demanding he answer to the charges, which will most likely be delivered in writing. They will also assign written trial briefs to the House managers and the president’s legal teams to be submitted next week.

• With that, the Senate will recess the trial until Tuesday, Jan. 21, to give lawmakers one last long weekend break.