Democrats are urging US Supreme Court nominee Amy Coney Barrett to recuse herself from any election-related cases because of President Donald Trump’s comments that he expects the justices to potentially decide the outcome, but there is no way to force her to do so.

Although US law requires justices to step aside when there is a conflict of interest or genuine question of bias, it leaves the individual justice to decide whether such a conflict exists. Aside from direct financial and personal conflicts, they rarely do so.

Trump on Saturday nominated Barrett to the vacancy created by the death of liberal Justice Ruth Bader Ginsburg on September 18. If confirmed by the Republican-controlled US Senate, Barrett would give the court a 6-3 conservative majority.

On Wednesday Trump said he wanted the full complement of nine justices on the court as soon as possible in part because he believes the court would determine the outcome of the November 3 presidential election. “I think this will end up in the Supreme Court, and I think it’s very important that we have nine justices,” he told reporters.

The Supreme Court has determined the outcome of a US presidential election only once, in 2000, leading President George W. Bush to the White House.

Trump indicated that the Supreme Court would rule in his favour with nine justices on board. He alleged they would respond to an unspecified “scam that the Democrats are pulling” in relation to increased use of mail-in ballots as a result of the pandemic.

Trump has repeatedly and without evidence attacked mail-in balloting, a longstanding feature of US elections.

Senate Democrats say they will probe Barrett on the subject during her confirmation hearing before the Senate judiciary committee in the hope she will pledge to step aside in certain cases.

“This will be hotly debated by members of the judiciary committee as well as legal ethicists but I suspect many of us will end up pressing that question in whatever form,” Democratic senator Chris Coons, a member of the committee, said. He added that “every standard of judicial conduct would suggest that where you have a complicated relationship, where you have arguably an ethical complication as a judge, you should recuse yourself”.

White House chief of staff Mark Meadows told CNN last week, before Barrett was officially named, that the nominee would not have to recuse herself and that the issue had not come up during interviews. During previous Senate hearings, Supreme Court nominees have routinely declined to commit on how they would approach cases that could come before them.