President Donald Trump's drive to cement the conservative grip on the U.S. Supreme Court faces a major test on Friday as the Senate holds a key procedural vote on Brett Kavanaugh, whose nomination for the country's highest court has set off a political brawl.

In the Senate, Trump's fellow Republicans were growing more confident they would win the 10:30 a.m. (1430 GMT) vote after two wavering Republican senators responded positively on Thursday to an FBI report on accusations of sexual misconduct against Kavanaugh.

The Senate, where Republicans hold a razor-thin majority, plans a final confirmation vote on Saturday.

The chairman of the Senate Judiciary Committee said Republican leaders were still not completely sure they had the votes needed to confirm Kavanaugh, Trump's second nominee to the court since he took office in January last year.

Asked on Fox News how the vote would go, Judiciary Committee Chairman Chuck Grassley, who oversaw Kavanaugh's contentious confirmation hearings, said, "As of now I don't really know and I don't know if anybody else does."

The divisive debate over Kavanaugh provoked demonstrations by thousands of protesters on Thursday. Opponents of the nominee rallied outside the Supreme Court and entered a Senate office building.

Senator John Cornyn, the No. 2 Senate Republican, said on Twitter on Friday that 302 protesters had been arrested for unlawfully demonstrating in Senate buildings.

Trump, himself accused by numerous women of sexual misconduct during the 2016 presidential race, wrote on Twitter that the FBI report showed that the allegations against Kavanaugh were "totally uncorroborated."

The confirmation of Kavanaugh, a federal appeals court judge, would tip the balance on the Supreme Court to a 5-4 conservative majority. He was nominated by Trump to succeed retired Justice Anthony Kennedy, who was seen as a swing vote on the court.

While she did not pledge support for Kavanaugh, moderate Republican Senator Susan Collins said the FBI probe appeared thorough.

Republican Senator Jeff Flake, who voted for Kavanaugh at the committee level on the condition that the FBI look more closely into the allegations against Kavanaugh, said he saw no additional information corroborating the accusations.

Most Democrats opposed Trump's nomination of Kavanaugh from the outset, but their objections sharpened when Christine Blasey Ford, now a college professor in California, and two other women accused Kavanaugh of sexual misconduct during the 1980s while he was a high school and college student.

Ford and Kavanaugh testified at a dramatic Senate Judiciary Committee hearing last week in which she described the alleged assault, and Kavanaugh denied all of the allegations against him, while accusing the Democrats of a political "hit."

His sometimes angry tone in turn drew accusations from critics that he was temperamentally unsuited and too overtly partisan to take up a place on the top court.