U.S. President Donald Trump announced on Saturday the nomination of conservative judge Amy Coney Barrett to the Supreme Court.
A staunch Catholic, Barrett has described abortion as "always immoral".
The Republican-dominated Senate is planning to confirm her in a vote before the election and the Democrats are powerless to stop it.
The court will then have a clear Republican majority, with six Republican-appointed justices to three appointed by Democrats.
The shift in the court's makeup - from Ruth Bader Ginsburg, a liberal icon, to an outspoken conservative - would be the sharpest ideological swing since Clarence Thomas replaced Justice Thurgood Marshall nearly three decades ago.
Within hours of Ginsburg's death, Trump had made clear his intention to nominate a woman, after previously putting two men on the court and as he struggles to mitigate erosion in support among suburban women.
He was said to be considering five women, but Barrett was at the White House at least twice this week, including for a Monday meeting with Trump. He is not known to have met with any of the other contenders.
Her nomination is the third judge that Trump has nominated to the Supreme Court in just one term in office.
Ginsburg was reported by her granddaughter to have said that "My most fervent wish is that I will not be replaced until a new president is installed."
The court has been dominated by five conservative-leaning justices but the appointment of Barrett will swing the court even further to the right.
Barret's avowed Catholicism is also politically advantageous for Trump as he seeks re-election.
Catholic voters in Pennsylvania, in particular, are viewed as a pivotal demographic in the swing state that Democratic nominee Joe Biden, also Catholic, is trying to recapture.