‘The president’s lawyer just rambled on and on.’ Trump defense lawyer leaves some senators scratching their heads.

After two hours of blistering opening statements from House Democrats prosecuting the impeachment case against former President Donald J. Trump, his defense lawyer, Bruce L. Castor Jr., opened with a slip, calling himself the “lead prosecutor.”

It didn’t get much smoother from there.

Mr. Castor, who is most famous for refusing to prosecute Bill Cosby for sexual assault when he was the district attorney in Montgomery County, Pa., started a meandering defense of Mr. Trump in which he rarely referenced the former president or his behavior on Jan. 6, when his supporters stormed the Capitol.

At times, Mr. Castor appeared to be arguing for Mr. Trump’s free speech rights and against a partisan cycle of impeachments. As he spoke, senators in the chamber sometimes appeared confused or uninterested.

At one point, Mr. Castor seemed to refer to Senator Ben Sasse, Republican of Nebraska, who has harshly criticized Mr. Trump for his actions on Jan. 6 and was one of only five members of his party to oppose an effort to dismiss the trial as unconstitutional. Mr. Castor called Nebraska a “judicial-thinking place” and suggested that Mr. Sasse “faces the whirlwind,” drawing a befuddled look from the senator.

As Mr. Castor spoke, other senators appeared restless and started talking among themselves.

“The president’s lawyer just rambled on and on,” said Senator John Cornyn, Republican of Texas. “I’ve seen a lot of lawyers and a lot of arguments, and that was not one of the finest I’ve seen.”

At one point, Senator Cory Booker, Democrat of New Jersey, stood up and leaned against the back wall of the chamber. It was a stark difference from moments before, when the lawmakers had sat rapt at their desks as Representative Jamie Raskin, Democrat of Maryland and the lead House impeachment manager, showed a harrowing video of the Capitol assault and closed with a tearful and deeply personal plea for justice.

Toward the end of his remarks, Mr. Castor got around to suggesting that the Senate should not go forward with the impeachment case, arguing that if Mr. Trump committed a crime, the Justice Department should simply arrest him.

“The American people just spoke — they just changed administrations,” said Mr. Castor, who is part of a hastily assembled legal team that stepped in when Mr. Trump parted ways with his original lawyers.

On the conservative TV station Newsmax, Alan M. Dershowitz, who served on Mr. Trump’s defense team during his first impeachment trial last year, panned Mr. Castor’s performance.

“I have no idea what he’s doing,” Mr. Dershowitz said. “Maybe he’ll bring it home, but right now, it does not appear to me to be effective advocacy.”

David I. Schoen, another of Mr. Trump’s impeachment lawyers who followed Mr. Castor, had an easier time keeping senators’ attention. He derided the House managers for hiring what he called a “movie company” to stitch together the most disturbing scenes of the Capitol attack as if it were a “blood sport.” Mr. Schoen played his own video presentation — with sinister music — showing clips of Democratic lawmakers through the years calling for Mr. Trump to be impeached.

And he offered a clear rationale for why a former president should not stand trial, arguing that it would set a dangerous precedent where any former official could be punished after leaving office for having carried out his or her duties.

“Under their unsupportable constitutional theory and tortured reading of the text,” Mr. Schoen said, “every civil officer who has served is at risk of impeachment if any given group elected to the House decides that what was thought to be important service to the country when they served now deserves to be canceled.”