US President Donald Trump has announced conservative appellate judge Amy Coney Barrett as his third US Supreme Court appointment, setting off a scramble in the Republican-led Senate to confirm her before election day in just over five weeks.

Ms Barrett appeared at the White House with Mr Trump as he made the announcement yesterday.

"Today it is my honour to nominate one of our nation's most brilliant and gifted legal minds to the Supreme Court," said the US President.

Turning to Ms Barrett, he said that "you are very eminently qualified for the job". She and her lawyer husband have seven children, two of whom were adopted from Haiti, and they joined her at the ceremony in the White House Rose Garden.

If confirmed to replace liberal icon Ruth Bader Ginsburg, who died at age 87 on 18 September, Ms Barrett would become the fifth woman ever to serve on the top US judicial body and push its conservative majority to a commanding 6-3.

With Mr Trump's fellow Republicans controlling the Senate, confirmation appears certain, though Democrats may try to make the process as difficult as possible.

"I love the United States and I love the United States Constitution," Barrett said, adding that she was "deeply honoured" in the confidence that the president had shown in her.

Senate Majority Leader Mitch McConnell issued a statement earlier praising the nomination.

"Judge Amy Coney Barrett is an exceptionally impressive jurist and an exceedingly well-qualified nominee to the Supreme Court of the United States," said Mr McConnell.

Barrett, 48, was appointed by Mr Trump to the Chicago-based 7th US Circuit Court of Appeals in 2017 and is a favourite of religious conservatives, a key Trump voter bloc.

Conservative activists have hailed the US president's selection, while liberals have voiced dismay.

Like Mr Trump's two other appointees, Neil Gorsuch in 2017 and Brett Kavanaugh in 2018, Ms Barrett is young enough that she could serve for decades. She is the youngest Supreme Court nominee since conservative Clarence Thomas was 43 in 1991.

The White House ceremony was decorated with American flags arranged in a way similar to the day when President Bill Clinton nominated Ms Ginsburg in 1993.

The selection kicks off a flurry of activity that must take place before the final confirmation vote, including public hearings in the coming weeks before the Senate Judiciary Committee.

Democrats are still furious over Senate majority leader Mitch McConnell's 2016 refusal to consider President Barack Obama's Supreme Court nomination of Merrick Garland because it came during an election year.

Democratic presidential candidate Joe Biden has said the winner of the election should get to replace Ruth Bader Ginsburg.

Republicans hold a 53-47 Senate majority. Only two Republican senators have opposed proceeding with the confirmation process.

Abortion rights advocates have voiced concern that Ms Barrett, a devout Catholic, could cast a vote for overturning the landmark 1973 Roe v. Wade ruling legalising abortion across the US.

They announced plans for a protest against her nomination today in front of the Supreme Court.

Ms Barrett has staked out conservative legal positions in three years on the bench, voting in favour of one of Mr Trump's hardline immigration policies and showing support for expansive gun rights.

She also authored a ruling making it easier for college students accused of campus sexual assaults to sue their institutions.

Born in New Orleans, Barrett received her law degree from Notre Dame Law School, a Catholic college in Indiana.

Election-related cases

President Trump has said he wants his nominee confirmed before the election so she would be able participate in any election-related cases that reach the justices, potentially casting a key vote in his favour.

A US presidential election's outcome only once has been determined by the Supreme Court, in 2000 when it clinched Republican George W Bush's victory over Democrat Al Gore.

Trump has repeatedly - without evidence - said voting by mail, a regular feature of American elections, will lead to voter fraud. He also has refused to commit to a peaceful transfer of power should he lose the election.

This marks the first time since 1956 that a US president has moved to fill a Supreme Court vacancy so close to an election.

In that year, President Dwight Eisenhower three weeks before winning re-election placed William Brennan on the court using a procedure called a "recess appointment" that bypassed the Senate, a tactic no longer available for installing justices.

An emboldened Supreme Court conservative majority could shift the United States to the right on hot-button issues by, among other things, curbing abortion rights, expanding religious rights, striking down gun control laws, and endorsing new restrictions on voting rights.