US Senators were set to hold an initial vote Friday on Brett Kavanaugh's nomination to the Supreme Court as President Donald Trump accused liberal financier George Soros of bankrolling opposition to the nominee.

Protesters pressured key lawmakers over the looming too-close-to-call vote, with three key Republican lawmakers still officially undecided on the nominee, who has faced accusations of sexual misconduct.

Trump took the brutal battle to a new stage when he dismissed women who have cited their own experiences of sexual assault to argue against Kavanaugh as "elevator screamers" and said Soros, a frequent target of conservatives, was behind their demonstrations.

"The very rude elevator screamers are paid professionals only looking to make Senators look bad. Don't fall for it!" he said on Twitter.

"Also, look at all of the professionally made identical signs. Paid for by Soros and others. These are not signs made in the basement from love!"

The tweet stoked tensions over Kavanaugh just hours ahead of a Senate cloture vote to end debate on the nomination and move to a final vote on Saturday.

The vote is expected after 10:30 am (1430 GMT).

Republicans continued to reject sexual assault and abuse allegations against Kavanaugh from three women when they were students in the 1980s, and dismiss charges from Democrats that Kavanaugh repeatedly lied in Senate testimony about his background.

With their party holding a slim 51-49 majority in the Senate, all eyes were focused on three Republican senators who have not yet declared what they will do in the final vote -- Jeff Flake of Arizona, Susan Collins of Maine and Lisa Murkowski of Alaska.

If two break ranks with their party over Kavanaugh, the crucial nomination could be sunk in a major defeat for Trump.

Collins early Friday said she would vote for cloture, but would not say yet where she stood on the final vote.

If he wins approval, though, Kavanaugh would seal a conservative majority on the nine-seat court for many years to come.

Gripping political battle

The confirmation process has gripped Washington and the nation, aggravating already deep political divisions with just weeks to go before mid-term congressional elections.

The 53-year-old appeals court judge made a last-minute pitch for support in a Wall Street Journal op-ed headlined "I am an independent, impartial judge," virtually unheard-of campaigning for a nominee to the high court.

Protesters returned outside the Capitol Friday morning, a day after 302 were arrested and charged with unlawfully demonstrating inside the Senate complex.

An MSNBC report said demonstrators were also outside the home of Senate leader Mitch McConnell early Friday, pretending to drink beer -- a reference to allegations that Kavanaugh was a heavy drinker while in high school and college, when the sex abuse is alleged to have taken place.

Senate Judiciary Committee chairman Chuck Grassley opened the Senate Friday ahead of the vote attacking an alleged Democratic political plot to defeat Kavanaugh, accusing unnamed "left-wing dark money groups" of engineering the opposition.

"The resistance that has existed since the day after the 2016 election is centered right here on Capitol Hill," he said. "I hope we can say no to mob rule by voting to confirm Judge Kavanaugh."

Trump's reference to Soros, who has supported pro-democracy movements around the world and the US Democratic Party for years, appeared to aim at inciting more support and anger from the president's conservative Christian base.

The Jewish billionaire is frequently cited by arch-conservatives as behind liberal and progressive movements -- criticisms that have raised counter-accusations of anti-semitism.

But Democrats continued Friday to argue that there had been too little effort made to investigate the allegations against Kavanaugh -- especially that of Christine Blasey Ford, who says a drunken Kavanaugh tried to rape her in 1982.

On Thursday the FBI released the results of a six-day, last-minute review of the charges which reportedly finds no corroborating evidence for the charges.

Democrats say the White House and Republicans kept the probe tightly limited in scope, preventing the FBI from interviewing key witnesses.

Ahead of the cloture vote Friday, another complication for Republicans loomed. Senator Steve Daines of Montana announced Thursday night that he has a scheduling conflict: his daughter is getting married Saturday in the western state and he plans to attend.

That could be bad news as Republicans need every vote they can get. McConnell's office said it is trying to figure out what to do.