The death of a Supreme Court justuce has prompted Republican senators to say they will block any new nomination by Barack Obama
The high-stakes judicial appointment to the nation's highest court has sparked a bitter battle of words between Democrats and Republicans.
Before Justice Antonin Scalia's death on Saturday, the US high court had a conservative 5-4 majority.
As a result, Republicans are threatening to use their dominance in the law-passing Senate to delay any appointment and prevent a ninth nomination from the tipping the balance in favour of the Democrats.
They fear a liberal "takeover" of the important legislative court and so are claiming the position ought to be left empty until after the presidential election.
The majority leader of the Senate, Mitch McConnell, has warned that the Republican-controlled body will block President Obama from fulfilling his recent promise to appoint a new justice himself.
"The American people should have a voice in the selection of their next Supreme Court Justice. Therefore, this new vacancy should not be filled until we have a new President."
But his opposition to the President has provoked a sharp response from Democrats in an issue likely to further polarise the presidential race for the White House.
1. What is the Supreme Court?
The Supreme Court, based in Washington DC, is established in the country's constitution as the court in which all "the judicial power of the United States shall be vested."
It takes on cases which appear to effect the nation as a whole, with its rulings setting a precedent for all lower courts. One of its best-known powers is to declare an act as a violation of the constitution.
2. Who are the justices?
Nine justices sit on the Supreme Court, with the weight until now in Republicans' favour 5-4 - that is, with five justices from the GOP and four Democrat justices.
According to the consitution, Congress - the law-making arm of government, divided into the House of Representatives and the Senate - is permitted to decide how to organise the Supreme Court.
But Article Two of the constitution also gives the President the power to appoint justices.
"He shall nominate, and by and with the Advice and Consent of the Senate, shall appoint ... Judges of the supreme Court," says the constitution.
3. Why is Justice Scalia's death so significant?
Scalia has left a gap which, if filled by a Democrat, could see Supreme Court power swing back towards a liberal viewpoint at 5-4 Democrats to Republicans.
This would at last give Democrats an arena of influence outside the White House. Currently both the House of Representatives, which is the shorter-term, law-making house of government, and the Senate, which is the longer-term law-making house, are controlled by Republicans.
Because of the non-proportional voting system in the US, the Republicans were able to claim more seats despite a Democrat presidential victory in 2014.
If the Republicans were able to block the Supreme Court appointment by Obama, by January 2017 they could potentially have a Republican-controlled House of Representatives, Senate and White House.
4. Can the Senate really block President Obama's appointment of a new Supreme Court Justice?
The law around this is hazy and tends to be brought up when political advantage suits one side or another.
Russell Wheeler, a judicial expert with the Brookings Institution, told the Washington Post: "It's just sort of a pie-in-the-sky flexibility that both parties try to disown when it's convenient for them and try to say it means something when it's not."
But because Republicans have control of the Senate, they can theoretically just refuse to pass Mr Obama's nomination.
Indeed, Ted Cruz, Marco Rubio and Donald Trump have already all urged their Republican allies in the Senate, and Mr McConell especially, to block any choice of justice by Mr Obama.
Both the Democrat minority leader Harry Reid, however, and Hillary Clinton, have said it is the President's right.
Senator Reid has said: "Failing to fill this vacancy would be a shameful abdication of one of the Senate’s most essential Constitutional responsibilities."