Top conservative lawyer says Trump can stand trial, in break with G.O.P. argument.

One of Washington’s leading conservative constitutional lawyers publicly broke on Sunday with the main Republican argument against convicting former President Donald J. Trump in his impeachment trial, asserting that a former president can indeed be tried for high crimes and misdemeanors.

In an opinion piece posted on The Wall Street Journal’s website, the lawyer, Charles J. Cooper, who is closely allied with top Republicans in Congress, dismissed as illogical the claim that it is unconstitutional to hold an impeachment trial for a former president. The piece came two days before the Senate was set to start the proceeding.

Since the rampage, Republicans have made little effort to excuse Mr. Trump’s conduct, but have coalesced behind the legal argument about constitutionality as their rationale for why he should not be tried, much less convicted. Their theory is that because the Constitution’s penalty for an impeachment conviction is removal from office, it was never intended to apply to a former president, who is no longer in office.

Many legal scholars disagree, and the Senate has previously held an impeachment trial of a former official — though never a former president. But 45 Republican senators, including Mitch McConnell of Kentucky, the minority leader who is said to believe that Mr. Trump committed impeachable offenses, voted last month to dismiss the trial as unconstitutional on those grounds.

Mr. Cooper said they were misreading the Constitution.

“The provision cuts their interpretation,” he wrote. He argued that because the Constitution allows the Senate to bar officials convicted of impeachable offenses from holding public office again in the future, “it defies logic to suggest that the Senate is prohibited from trying and convicting former officeholders.”

Mr. Cooper’s decision to take on the argument was particularly significant because of his standing in conservative legal circles. He was a close confidant and adviser to Senate Republicans, like Ted Cruz of Texas when he ran for president, and represented House Republicans — including the minority leader, Representative Kevin McCarthy of California — in a lawsuit against Speaker Nancy Pelosi.

But Mr. Cooper, who is said to be dismayed by the unwillingness of House and Senate Republicans to hold Mr. Trump accountable, took on the main claim made by his own confidants and clients, offering a series of scholarly and technical arguments for why the Constitution allows for a former president to stand trial.

Nicholas Fandos contributed reporting.