More than 1200 US law professors have signed a letter saying that Supreme Court nominee Brett Kavanaugh displayed a lack of judicial restraint at a Senate hearing last week - behavior that would be disqualifying for any court nominee.

Kavanaugh was responding to accusations from a California professor, Christine Blasey Ford, that he sexually assaulted her at a house party when they were teenagers in the 1980s. At the hearing, he vehemently defended his innocence and derided what he called "a calculated and orchestrated political hit".

Afterward, law professors across the country began discussing "with great distress, the unprecedented and unfathomable demeanor of Judge Kavanaugh," said Bernard Harcourt, a professor at Columbia Law School.

The letter, which will be sent to the Senate, grew out of those conversations.

"It was a spontaneous reaction to the hearing," Harcourt said.

The groundswell was overwhelming, he said, with hundreds of lawyers from more than 150 law schools signing on within hours.

Signatories included Martha Minow - the former dean of Harvard Law School, where Kavanaugh taught a popular course - and some scholars who previously supported Kavanaugh.

Harcourt said they signed out of concern about a rush to judgment, in the belief that for the Senate to elevate Kavanaugh "without full information and deliberation to the Supreme Court would undermine the respect owed" to the institution.

The letter to the Senate cites two laws governing bias and recusal, noting, "Judges must step aside if they are at risk of being perceived as or of being unfair ...

"We have differing views about the other qualifications of Judge Kavanaugh," they wrote.

"But we are united, as professors of law and scholars of judicial institutions, in believing that Judge Kavanaugh did not display the impartiality and judicial temperament requisite to sit on the highest court of our land."