But Mr. Cooper seemed to be working to change minds. He wrote in his opinion piece that in the short time since the vote, legal experts’ understanding of the issue had evolved and “exposed the serious weakness of Mr. Paul’s analysis.”
“The senators who supported Mr. Paul’s motion,” he wrote, “should reconsider their view and judge the former president’s misconduct on the merits.”
The question of constitutionality could come to a head quickly when the trial opens on Tuesday. Though Senate leaders were still debating the precise structure of the trial, prosecutors and Mr. Trump’s defense team were preparing for the possibility that Mr. Paul or another senator could force a second vote on the question on the opening day, before either side gets into their full presentations.
Mr. Cooper has a deep and long history in the conservative legal movement. He grew up in Alabama, and despite not attending an Ivy League law school, landed a clerkship for Justice William H. Rehnquist in 1978 before he became the chief justice, and at a time when Justice Rehnquist was considered the most conservative member of the court.
Mr. Cooper went on to become the head of the Justice Department’s Office of Legal Counsel during the Reagan administration, writing several highly conservative and controversial interpretations of the law, including one about whether employers could decline to hire someone who may have AIDS.
As a private lawyer, he has defended issues like school prayer and was an active member in the Federalist Society. In 2010, when the Republican National Lawyers Association named him the Republican lawyer of the year, there were three speakers for Mr. Cooper: Mr. Bolton; the head of the N.R.A., Wayne LaPierre; and Ed Meese, an attorney general under Ronald Reagan who was considered among the most conservative in the department’s history.
In the early days of the Trump administration, Mr. Cooper — who is longtime friends with Mr. Sessions — was considered to be the solicitor general. But Mr. Cooper remained in private practice, becoming the lawyer for Mr. Sessions as he was enmeshed in controversy related to the Russia investigation. In the second half of the Trump presidency, Mr. Cooper represented Mr. Bolton and his deputy, Charles Kupperman, in the first impeachment trial of Mr. Trump.
Mr. Cooper has continued to represent Mr. Bolton as the Justice Department has sued him to recoup money he made from a damning book he published about Mr. Trump.
Nicholas Fandos contributed reporting.