As President Trump’s impeachment trial moves into the defense phases, attorney Alan Dershowitz on Sunday said that he has changed his mind on whether a crime is needed to remove a president from office -- a reversal of his stance during the impeachment of President Bill Clinton in 1999.

Dershowitz, who recently joined Trump’s impeachment defense team, argued that a crime needs to be committed to impeach a president – a 180-degree shift from his previous thinking – and added that even after lengthy arguments by the House managers last week, he still sees Democrats' arguments falling far short of swaying the Senate to remove Trump from office.

“The conduct has to be criminal in nature -- it can’t be abuse of power; it can’t be obstruction of Congress,” he said. “Those are precisely the arguments that the framers rejected.”

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The Harvard Law School professor said that he changed his thinking after doing more research on the matter. During an interview in the midst of the Clinton impeachment, Dershowitz said, “you don’t need a technical crime” to impeach a president.

“I did say that then, and then I’ve done all the extensive research,” Dershowitz said on Sunday. “I’ve been immersing myself in dusty old books, and I’ve concluded that no, it has to be crime.”

Dershowitz called the charges of abuse of power and obstruction of Congress, which the House impeached Trump on in December, “vague” and “open-ended.”

“Even if the factual allegations are true -- which are highly disputed and which the defense team will show contrary evidence -- but even if true, they did not allege impeachable offenses,” Dershowitz said on “Fox News Sunday.” “So there can’t be a constitutionally authorized impeachment.”

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Dershowitz is expected to argue next week that an impeachable offense requires criminal-like conduct, even though many legal scholars say that's not true.

Defense lawyers say Trump was a victim not only of Democratic rage but also of overzealous agents and prosecutors. One of Trump’s lawyers, Jay Sekulow, cited mistakes made by the FBI in its surveillance of a former Trump campaign aide in the now-concluded Trump-Russia election investigation, and referred to the multimillion-dollar cost of that probe.

The Senate is heading next week toward a pivotal vote on Democratic demands for testimony from top Trump aides, including Acting Chief of Staff Mick Mulvaney and former National Security Adviser John Bolton, who refused to appear before the House. It would take four Republican senators to join the Democratic minority to seek witnesses, and so far the numbers appear lacking.

The Associated Press contributed to this report.