One of the high-profile lawyers hired by Donald Trump to defend him in the Senate trial starting this week says he will argue that abuse of power is not an impeachable offence.
Mr Trump is accused of abuse of power and obstruction of Congress.
Last year the Democrat-controlled House of Representatives concluded that he had abused his office by pressuring the Ukrainian president Volodymyr Zelensky to announce investigations into political opponents including Joe Biden by withholding military aid and a White House meeting.
They said he also obstructed their investigation by stopping key staff from testifying and refusing to hand over documents.
The president insists he has done nothing wrong and that the case against him is a Democratic ploy to stop him winning re-election in November.
This week the Senate will begin a trial triggered by the impeachment. While in theory he could be removed from office, the Republican majority in the upper chamber is expected to acquit him.
Mr Dershowitz was named on Friday as one of several high-profile legal experts added to Mr Trump's defence team, along with Kenneth Starr, the special counsel who led the impeachment inquiry against President Bill Clinton in the late 1990s.
He told CNN's State of the Union show: "I will be advocating against impeachment of this president based on the constitutional criteria."
Mr Dershowitz cited Justice Benjamin Curtis -- who defended President Andrew Johnson during his impeachment in 1868 – in arguing that "criminal-like conduct" is required, an argument that he said prevailed in clearing the 17th president of the United States.
The Harvard Law emeritus professor told CNN: "I will be making that argument as a lawyer on behalf of the president's defence team against impeachment."
Speaking to ABC's This Week, he said Mr Trump should be cleared even if his behaviour was wrong. "If the allegations are not impeachable, then this trial should result in an acquittal, regardless of whether the conduct is regarded as OK by you or by me or by voters. That's an issue for the voters."
However, Adam Schiff, who is leading the case against Mr Trump for the Democrats, called the position "absurdist".
He told This Week: "You had to go so far out of the mainstream to find someone to make that argument. You had to leave the realm of constitutional law scholars and go to criminal defence lawyers.
"The logic of that absurdist position that's being now adopted by the president is he could give away the state of Alaska, he could withhold execution of sanctions on Russia for interfering in the last election, to induce or coerce Russia to interfere in the next one."