US President Donald Trump has urged Republican lawmakers to confirm his upcoming nomination for the Supreme Court "without delay".

It comes after the issue upended the election only a day after the death of liberal stalwart Ruth Bader Ginsburg.

The 87-year-old justice, immensely popular among Democrats, died on Friday after a long battle with cancer, prompting an outpouring of national grief.

Her death, coming just weeks before the 3 November election, leaves her seat on the court - to which justices are appointed for life - vacant, offering Republicans a chance to lock in a conservative majority at the court for decades.

The stakes are high as the decision could affect such life-and-death issues as abortion, healthcare, gun control and gay rights.

They are pushed even higher in a bitter election year when the justices can play a decisive role in legal wrangling over a contested result - such as when they ruled in George W. Bush's favor to end the 2000 election debacle.

There are nine justices on the court. Trump has already named two during his first term as president, Brett Kavanaugh and Neil Gorsuch, giving conservatives a 5-4 majority before Ginsburg's death, though that does not guarantee rulings in Trump's favor - there have been several recent examples of conservatives siding with their progressive colleagues to tilt the balance.

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President Trump, who is lagging in the polls behind Joe Biden, has another powerful incentive to move ahead: providing a jolt of enthusiasm among his anti-abortion and evangelical supporters.

"We have this obligation, without delay!" Trump tweeted.

But, with 45 days to go before the election and some early voting already begun in some states, Democrats are pushing back furiously.

Joe Biden said Friday that "the voters should pick the president and the president should pick the justice for the Senate to consider".

The prospect of a fierce partisan nomination battle and rushed Senate confirmation vote has ignited his party, still seething over Republicans preventing Barack Obama from filling a court vacancy through most of the 2016 election year.

"Democrats are hopping mad about this - not just a little mad," University of Virginia political scientist Larry Sabato told CNN.

'Nothing is off the table'

While Democrats' options seem limited, Senate minority leader Chuck Schumer told party members Saturday that if Republicans press ahead then "nothing is off the table," according to media reports.

"This vacancy should not be filled until we have a new president," Schumer said, carefully echoing the words of Senate Republican leader Mitch McConnell in 2016 when he blocked Obama nominee Merrick Garland.

Republicans in theory have the Senate votes to push through a Trump nominee, but they could be blocked by only a handful of defections.

Analysts predicted Democrats would do their best to drag out the process while fanning public outrage over what Democrats called the Republicans' hypocrisy.

"Their option is to build a groundswell... to try to convince at least four Republican senators to vote 'no' on whoever the president puts forward," Amy Howe, co-founder of a Supreme Court blog, said on CNN.

A confirmation vote of Trump's eventual nominee before 3 November would be unusually quick. The average period from nomination to confirmation is around 70 days.

Republican doubters

A handful of moderate Republican senators - including Lisa Murkowski of Alaska and Susan Collins of Maine - have already expressed doubts about a rushed vote.

Other Republicans facing tough reelection races may be loath to take a stand before the vote.