US President Donald Trump stood firmly by his Supreme Court pick Brett Kavanaugh Monday after allegations that the conservative judge sexually assaulted a woman when they were teenagers threatened to derail the nomination process.

Senate Judiciary Committee Chairman Chuck Grassley canceled a vote on Kavanaugh planned for Thursday and said the nominee and his accuser, California university professor Christine Blasey Ford, would both appear next Monday to testify under oath about the alleged incident.

Trump characterized the issue as a "little delay" and described as "ridiculous" the suggestion that Kavanaugh might withdraw his candidacy.

"Judge Kavanaugh is one of the finest people I have ever known," Trump told reporters from the Oval Office. "Never even had a little blemish on his record."

"I think he's on track, very much on track," Trump said.

Trump did not comment directly on Ford's claim that a drunken Kavanaugh and one of his friends trapped her in a room and attempted to pull her clothes off at a high school party in 1982.

Kavanaugh flatly denied the accusation and was, according to the White House, ready to testify "tomorrow" in his defense.

"I have never done anything like what the accuser describes -- to her or to anyone," he said in a statement.

Debra Katz, a lawyer for Ford -- who only came forward publicly on Sunday after trying to keep her name hidden for more than a month -- had said earlier she was ready to speak under oath before the Senate Judiciary Committee.

That could result in a tense replay of the lurid 1991 hearing on Clarence Thomas, the conservative Supreme Court justice accused of repeated sexual harassment by a former assistant, Anita Hill.

 #MeToo moment 

The explosive assault claim makes the Supreme Court the latest US institution to be rocked by fallout from the year-old #MeToo movement exposing sexual misconduct.

Over the past year, numerous politicians, businessmen and entertainment and media figures have been forced from their posts by accusations of harassment and rape by women, some dating back decades.

The allegation now threatens to inject a potentially heated "he said-she said" battle into the looming midterm congressional elections, with Republican control of the House of Representatives and the Senate at stake.

"Anyone who comes forward as Dr Ford has done deserves to be heard," Grassley said in a statement.

"To provide ample transparency, we will hold a public hearing Monday to give these recent allegations a full airing."

Moderate Republican Senator Susan Collins, whose support is crucial to Kavanaugh's nomination in the narrowly-divided Senate, said she needs to have both the judge and his accuser answer questions under oath.

"I don't know enough about Dr Ford and her allegations yet" to reach a conclusion, Collins said.

"Obviously, if Judge Kavanaugh has lied about what happened, that would be disqualifying."

The allegations also stand to derail one of the most consequential appointments to the nine-seat high court in decades.

Evenly balanced for years, the court is expected to tilt decidedly conservative if Kavanaugh is approved, potentially threatening women's access to abortion, restrictions on presidential powers, and efforts to curb gun ownership.

Underscoring the stakes, Judicial Crisis Network, a conservative lobby group led by a lawyer Carrie Severino, who once worked under Clarence Thomas, announced Monday it would spend $1.5 million on media advertising in support of Kavanaugh.

"We are not going to stand by and let Judge Kavanaugh be smeared. Countless women have attested to his exemplary personal and professional character throughout his life," said Severino.

 Accuser feared public scrutiny 

Trump questioned why the accusation became public only on Friday, after Kavanaugh had undergone four days of questioning in early September.

Katz told US media her client had been ambivalent about the pressure that would come with speaking out publicly.

She first contacted lawmakers about her experience in July, but initially insisted on remaining remain anonymous, saying the 1982 incident, when she and Kavanaugh both attended private schools in suburban Washington, had been a source of lasting trauma.

"She's now going to have to live with the tremendous efforts by people to annihilate her and to try to discredit her," Katz told CNN.