Lev Parnas, a former associate of Rudolph W. Giuliani who has offered to testify at the impeachment trial, arrived in Washington on Wednesday to show support for what his lawyer called “a fair trial.”

Mr. Parnas, who flew in from Florida and met his lawyer as well as a swarm of reporters at Washington Union Station near the Capitol, said he was in town to “look at the senators and have them look at me and see that we need to call witnesses. The truth needs to come out. I’m here.”

Mr. Parnas was involved in the campaign to pressure Ukraine and described himself as “one of the most important witnesses.”

The cameras then followed Mr. Parnas as he walked toward the Capitol.

“It’s funny, I’ve been to Washington,” Mr. Parnas said, an apparent reference to the many Republican donor events he has attended in the city and his involvement in meetings to plan the pressure campaign. And yet, he said, “This is my first time being out of the Trump hotel.”

As he neared the Capitol, a small crowd of supporters chanted, “Let Lev speak.”

Alongside his lawyers, Mr. Parnas stopped to snap selfies with some of the protesters on his way to the office of Senator Chuck Schumer, Democrat of New York and the minority leader, where he picked up passes for the impeachment trial.

Mr. Parnas, who was charged in October with federal campaign finance crimes, cannot attend the trial because he is wearing an ankle-monitoring device as a condition of his bail. His lawyer, Joseph A. Bondy, planned to attend.

“How do you get to the truth if you don’t have witnesses?” Mr. Bondy said.