President Donald Trump’s defence against impeachment relies in part on arguments made in the precedent-setting trial of President Andrew Johnson more than 150 years ago: that impeachment requires a crime.

But most legal scholars disagree, including a law professor called by Republicans in the House investigation to argue against impeaching Mr Trump.

A lawyer for Johnson argued in his opening statement to the Senate that Johnson could not be removed from office because he was not guilty of a crime. Johnson was acquitted by a single vote.

One of Mr Trump’s lawyers, Alan Dershowitz says that same argument — that impeachment requires “criminal-like conduct” — will be central to the constitutional defence he will make on the president’s behalf.

The idea may be attractive to Republicans seeking a legal basis to acquit Mr Trump of having abused his power and obstructing Congress.

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President Donald Trump, whose legal team argues he can not be impeached without having clearly committed a crime (Manuel Balce Ceneta/AP)

But legal scholars dispute the idea that the Founding Fathers ever intended for impeachable offences to require proof of a crime.

And historians are equally doubtful that the argument from Johnson’s lawyer, Benjamin Robbins Curtis, can be credited with securing Johnson’s narrow acquittal.

“This is a way in which history is weaponised and distorted in order to give these kinds of arguments heft,” said Rachel Shelden, a Penn State University history professor and Civil War-era expert.

“It’s a way of trying to promote an understanding of the Johnson impeachment that is false, based on what historians now believe.”

At issue is the Constitution’s standard for impeachment: “treason, bribery, or other high crimes and misdemeanours”.

Down the years, the threshold has been understood to encompass actual crimes — judges have been impeached for sex abuse and soliciting bribes, among other offences — but also non-criminal misconduct such as being drunk on the bench or favouritism in the appointment of bankruptcy receivers.

Johnson was impeached over accusations he violated the Tenure of Office Act, which barred presidents from firing certain officials without Senate approval, over his removal of War Secretary Edwin Stanton.

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An artist’s sketch depicting impeachment manager, Representative Adam Schiff, speaking at the impeachment trial on Wednesday (Dana Verkouteren/AP)

Johnson’s defence team questioned the constitutionality of that now-extinct law, and at his 1868 trial one of his lawyers asserted in his opening argument that an impeachable offence “refers to, and includes only, high criminal offences against the United States”.

“There can be no crime, there can be no misdemeanour without a law, written or unwritten, express or implied,” said Curtis, a former Supreme Court Justice.

Mr Dershowitz, who is expected to make a constitutional presentation to the Senate within days, said on CNN’s “State of the Union” on Sunday he would paraphrase Curtis’s argument that “the framers intended for impeachable conduct only to be criminal-like conduct or conduct that is prohibited by the criminal law”.

But legal scholars and Democrats are decrying the claim that an impeachable offence must be an indictable crime.

Jonathan Turley, a George Washington University law professor who argued to the House against impeachment, wrote in The Washington Post on Wednesday that such an argument was politically unwise and constitutionally shortsighted.

“They had to go outside the realm of constitutional lawyers and scholars to a criminal defence lawyer to make that argument because no reputable constitutional law expert would do that,” said California Representative Adam Schiff, chairman of the House Intelligence Committee. He is one of the seven Democratic managers prosecuting the impeachment case.

Democrats say Mr Trump’s alleged abuse of power — pressing Ukraine for an investigation into a Trump Democratic rival, Joe Biden, at the same time his administration withheld millions of dollars in military aid — is a “quintessential impeachable offence”.

They say the Founding Fathers intentionally created a flexible standard of acts that could result in impeachment, and that Mr Trump’s actions in this instance are the framers’ “worst nightmare”.