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(CNN) —  

A wide array of Senate Republicans harshly criticized former President Donald Trump’s defense team on the opening day of his second impeachment trial, arguing that Trump attorney Bruce Castor had delivered a rambling and unfocused argument in making the case that the proceedings are unconstitutional.

“I thought the President’s lawyer, the first lawyer, just rambled on and on and on,” John Cornyn of Texas, a member of Senate GOP leadership, said of Castor, adding that the lawyer “didn’t really address the constitutional argument. Finally the second lawyer got around to it, and, I thought, did an effective job.”

Castor opened Trump’s defense with a meandering presentation and warned that a second impeachment trial in 13 months would “open the floodgates” to future impeachments, even making the rhetorically unfounded suggestion that former Obama administration Attorney General Eric Holder could be impeached.

Prominent Republicans argued on Tuesday that it was hard to understand exactly what Castor’s argument was, while emphasizing that they thought attorney David Schoen was more skilled in his role.

GOP Sen. Lisa Murkowski of Alaska said, “I was really stunned at the first attorney who presented for former President Trump. I couldn’t figure out where he was going, spent 45 minutes going somewhere, but I don’t think he helped with us better understanding where he was coming from on the constitutionality of this. And I felt that Mr. Schoen … did a better job, but I think they sure had a missed opportunity with their first, first attorney there.”

GOP Sen. Susan Collins of Maine was also critical of Castor’s performance, telling CNN, “I thought the second lawyer … made the arguments very well. I was perplexed by the first attorney, who did not seem to make any arguments at all, which was an unusual approach to take.”

The criticism is a stinging rebuke to Trump and his defense team from members of his own party, and may reflect a growing willingness on the part of Republicans to criticize the former President in the wake of the Capitol attack he provoked, even as it looks all but certain that he will be acquitted at the end of the trial by Senate Republicans.

While many Republicans took issue with Castor, they were quick to praise Rep. Jamie Raskin, a Maryland Democrat serving as the lead House impeachment manager and arguing the case against Trump.

GOP Sen. Kevin Cramer said that Raskin did a “superior job” and the Trump team was “not very well prepared” – echoing other GOP senators.

“I don’t think the lawyers did the most effective job,” said GOP Sen. Ted Cruz of Texas when asked about Trump’s lawyers. He added that Raskin was “impressive” and a “serious lawyer.”

GOP Sen. Bill Cassidy of Louisiana said that “anyone who listened to President Trump’s legal team saw they were unfocused, they attempted to avoid the issue. And they talked about everything but the issue at hand.”

Castor struck an optimistic tone in the face of mounting criticism from GOP senators, telling reporters, “I thought we had a good day.”

He did not engage when a reporter asked him to address the growing list of Republicans who had criticized his time on the Senate floor Tuesday and instead reiterated, “I thought we had a good day, thank you.”

Schoen defended his colleague, saying, “I thought he did a great job,” when asked about his colleague’s arguments.

Cramer said the argument from Castor was “a little disorganized” and that “the problem was it was more spontaneous than it was planned and prepared.”

Cramer, who voted against the constitutionality of the impeachment trial, said he thought Trump’s defense team was caught off-guard by the House impeachment managers’ arguments, saying, “I, frankly, think they were a little surprised, to be honest.”

“I think it got him off to, frankly, a pretty rocky start,” Cramer said.

This story has been updated with additional developments Tuesday.

CNN’s Ali Zaslav, Aaron Pellish and Jeremy Herb contributed to this report.