Democrats are raising new questions about the truthfulness of US supreme court nominee Brett Kavanaugh’s sworn testimony to the US senate.

The move marks a shift in tactics against US president Donald Trump’s choice for the highest court in America as all sides await the results of the FBI’s background investigation into sexual misconduct allegations.

Chuck Schumer, the senate Democrats’ leader, accused Mr Kavanaugh of delivering a “partisan screed” during a judiciary committee hearing last week.

He said Mr Kavanaugh seemed willing to “mislead senators about everything from the momentous to the mundane” to ensure his ascension to the court.

Mr Schumer said: “The harsh fact of the matter is that we have mounting evidence that Judge Kavanaugh is just not credible.”

Chuck Schumer
Senate minority leader Chuck Schumer raised concerns over Mr Kavanaugh’s testimony (AP)

However, Senate Republican leader Mitch McConnell said the Democrats are simply looking to “move the goalposts” to prevent Mr Kavanaugh’s confirmation.

He pledged that the full US senate would begin voting on Mr Kavanaugh’s nomination this week.

Mr McConnell said: “The time for endless delay and obstruction has come to a close.”

Mr Kavanaugh’s confirmation hinges on a handful of key Republican and Democratic senators who have not yet fully tipped their votes.

One of them is Republican Jeff Flake of Arizona, who was greeted by hundreds of liberal protesters, victims of sexual assault among them, during an appearance on Monday in Boston.

Mr Flake and senators Susan Collins of Maine and Lisa Murkowski of Alaska were instrumental last week in holding up Mr Kavanaugh’s confirmation vote.

Jeff Flake
Senator Jeff Flake (AP)

They forced the White House to open a supplemental background investigation of sexual misconduct allegations against the judge.

The votes of the three Republicans and those of red-state Democrats Joe Manchin of West Virginia and Heidi Heitkamp of North Dakota will largely determine whether Mr Kavanaugh is confirmed.

Mr Flake said he would ensure it is a “real investigation” by the FBI as Democrats demanded that Mr Trump give them a full readout of his instructions to the agency.

“It does us no good to have an investigation that just gives us more cover,” Mr Flake said.

Ms Heitkamp, meanwhile, noted that Mr Trump himself called for a “broader” FBI investigation into the allegations.

She said: “I’m waiting to see what the results are.”

Mr Kavanaugh has emphatically denied Christine Blasey Ford’s allegation that he sexually assaulted her at a gathering when they were teenagers.

He has also denied an accusation from Deborah Ramirez, a classmate at Yale, who said he exposed himself to her at a dorm party more than 25 years ago.

A third claim – from Julia Swetnick, who is represented by attorney Michael Avenatti – accuses Mr Kavanaugh of excessive drinking and inappropriate treatment of women at parties in the early 1980s. Mr Kavanaugh denies that as well.

Democrats have seized most recently on Mr Kavanaugh’s indignant, emotional testimony before the judiciary committee to question whether he has the temperament for a lifetime appointment on the nation’s highest court.

In particular, Democrats have homed in on his contention that the allegations against him are an “orchestrated political hit” funded by left-wing groups seeking “revenge on behalf of the Clintons”.

Senator Mazie Hirono, a Democrat from Hawaii, asked: “We’re going to put a conspiracy theory believer on the court?”

Democrats are also questioning Mr Kavanaugh’s honesty, particularly over statements he made about his drinking in high school and college after a former classmate at Yale said he was a “frequent, heavy drinker”.