Republicans have secured the numbers needed to ensure that President Donald Trump's Supreme Court nominee will face a confirmation vote in the Senate.
Senator Mitt Romney of Utah has given the party the 51 backers needed to move forward with voting on Mr Trump's candidate to replace Ruth Bader Ginsburg, who died on Friday.
Democrats argued there should be no confirmation in an election year.
The move guarantees a bitter political battle going into November's vote.
President Trump says he will announce his chosen nominee on Saturday at 17:00 local time (22:00b), and has vowed to pick a woman.
Supreme Court justices are nominated to the bench by the US president, but must be approved by the Senate.
With the death of Justice Ginsburg, a liberal stalwart, Mr Trump has been given the chance to cement a rightward ideological tilt of the nine-member court by replacing her with a conservative.
Senate majority leader Mitch McConnell has vowed to hold a confirmation vote before the election in November, but a question mark had hung all week over whether enough Republicans in the chamber would back him.
Although they hold a slim majority with 53 seats, two centrist Republican senators - Susan Collins of Maine and Lisa Murkowski of Alaska - said they were sceptical of confirming a lifetime judicial appointment in an election year.
On Tuesday, Ms Collins told reporters she would vote "no", saying the Senate ought to follow the "same set of rules" it used in 2016 to block then-President Barack Obama's top court nominee. Ms Collins is facing a tough re-election bid this year.
Mr Romney, a Trump critic who the president called "our worst senator" earlier this month, was seen as a possible defector. Mr Romney is one of the few Republicans in Washington willing to criticise Mr Trump in public and voted earlier this year to convict him during his impeachment trial.
However, in a statement released on Tuesday, Mr Romney said he would give Mr Trump's nominee a hearing, citing "historical precedent".
"My decision regarding a Supreme Court nomination is not the result of a subjective test of 'fairness' which, like beauty, is in the eye of the beholder," he said.
"It is based on the immutable fairness of following the law, which in this case is the Constitution and precedent. The historical precedent of election year nominations is that the Senate generally does not confirm an opposing party's nominee but does confirm a nominee of its own."
Republicans fall into line
It didn't take long for Republican senators to fall into line.
In the end, concerns about hypocrisy - just four years ago the Republican majority blocked Barack Obama's Supreme Court nomination because it was made in "an election year" - took a backseat to simple power politics. The Republicans have the opportunity to cement a solid conservative majority on the high court, and they won't let it slip away - even if there are political consequences for Republican senators seeking re-election in moderate states.
Democrats will howl with anger, but at this point there's not much they can do, procedurally, to stop what seems inevitable. Instead, they will issue warnings of grave consequences if and when they take power next year. They've threatened to add seats to the Supreme Court, admit new, Democratic-leaning states (namely, Washington DC and Puerto Rico) or strengthen the power of the Senate majority. All those are hypothetical battles for another day, however.
For the moment, the Republicans are relishing the prospect of the biggest swing in the ideological makeup of the court in three decades - with issues like abortion, voting rights, healthcare, gun control and civil liberties hanging in the balance.
What is the historical precedent?
Since Ginsburg's death, Republican senators have been fending off accusations of hypocrisy on approving a Supreme Court justice during an election year.
In 2016, Mr McConnell refused to hold hearings for Democratic President's Barack Obama's nominee for the high court, Merrick Garland.
The nomination, which came 237 days before the election, was successfully blocked because Republicans held the Senate and said the decision should be made outside of an election year. With 42 days before the 2020 election, Democrats now say the Republicans should stand by their earlier position and let voters decide.
"I want you to use my words against me," South Carolina Republican Lindsey Graham said in 2016. "If there's a Republican president [elected] in 2016 and a vacancy occurs in the last year of the first term, you can say Lindsey Graham said, 'Let's let the next president, whoever it might be, make that nomination.'"
Yet Mr Graham, chairman of the powerful Judiciary committee that first vets a candidate, has said he will "be leading the charge" to push Mr Trump's candidate forward this year.
It is rarer to confirm a Supreme Court nominee when the White House and Senate are held by different parties than when they are both of the same party, but approvals do happen.
In 1968, Democratic President Lyndon Johnson failed to get his nominee for Supreme Court chief justice - Abe Fortas - approved by the Democratic-controlled Senate after it was blocked by the Republican minority and conservative Democrats who disagreed with liberal positions he had taken as a Supreme Court associate justice and his close personal ties to the president.
In a speech on Monday, Mr McConnell referred to that moment in history, saying: "Apart from that one strange exception, no Senate has failed to confirm a nominee in the circumstances that face us now" - referring to the situation where the president and the majority in the Senate are of the same party.
"The historical precedent is overwhelming and it runs in one direction. If our Democratic colleagues want to claim they are outraged, they can only be outraged at the plain facts of American history."