By Lawrence Hurley and Richard Cowan

WASHINGTON (Reuters) - Republicans failed on Thursday to end a Democratic bid to block a U.S. Senate confirmation vote on President Donald Trump's Supreme Court nomination but were poised to quickly resort to a rule change dubbed the "nuclear option" to allow approval of Neil Gorsuch a day later.

With ideological control of the nation's highest court at stake in the fierce political showdown, the Senate held a vote to end debate on Gorsuch's nomination and move toward a Friday vote to confirm him to the lifetime post.

But with the 55-45 tally, Republicans fell short of the 60-vote super-majority needed to overcome the Democratic procedural tactic called a filibuster and proceed to a vote in which senators could confirm him by a simple majority.

Republicans were set to immediately move to hold a vote on changing long-standing Senate rules in order to prohibit filibusters against Supreme Court nominees.

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The rule change, which requires a simple majority, has been dubbed the "nuclear option" because it has been considered an extreme break with Senate traditions, and Trump has encouraged McConnell to "go nuclear." Republicans control the Senate 52-48.

Republicans said Gorsuch would be confirmed on Friday one way or the other.

"This will be the first and last partisan filibuster of the Supreme Court," Republican Senate Majority Leader Mitch McConnell said on the Senate floor on Thursday ahead of the vote.

"In 20 or 30 or 40 years, we will sadly point to today as a turning point in the history of the Senate and the Supreme Court, a day when we irrevocably moved further away from the principles our founders intended for these institutions: principles of bipartisanship, moderation and consensus," Senate Democratic leader Chuck Schumer said on the Senate floor.

Senate confirmation of Gorsuch, 49, would restore the nine-seat court's 5-4 conservative majority, enable Trump to leave a lasting imprint on America's highest judicial body and fulfill a top campaign promise by the Republican president.

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Republicans have called him superbly qualified for the job and one of the most distinguished appellate judges on the bench, and they blamed the Democrats for politicizing the confirmation process.

Democrats accuse Gorsuch of being so conservative as to be outside the judicial mainstream, favoring corporate interests over ordinary Americans in legal opinions, and displaying insufficient independence from Trump.

"This isn't really about the nominee anyway," McConnell said. "The opposition to the this particular nominee is more about the man who nominated him and the party he represents than the nominee himself."

Democrats expressed anger that the Senate, under McConnell's guidance, refused last year to consider Democratic former President Barack Obama's nomination of appellate judge Merrick Garland to fill the same high court vacancy that Trump elected Gorsuch to fill. The nine-seat Supreme Court has had a vacancy since conservative Justice Antonin Scalia died in February 2016.

'THE FALLOUT'

"The nuclear option was used by Senator McConnell when he stopped Merrick Garland. What we face today is the fallout," Democratic Senator Richard Durbin said on the Senate floor.

What Republicans did to Obama's nominee Garland was worse than a filibuster, Schumer said. Schumer said Republicans denied "the constitutional prerogative of a president with 11 months left in his term."

McConnell blamed the escalation of fights over judicial nominees on the Democrats and their opposition starting three decades ago to nominees made by Republican former Presidents Ronald Reagan and George W. Bush.

McConnell called the Democratic effort against Gorsuch "another extreme escalation in the left's never-ending drive to politicize the court and the confirmation process." He accused Gorsuch's opponents of "a singular aim: securing raw power no matter the cost to the country or the institution."

The 60-vote threshold that gives the minority party power to hold up the majority party has forced the Senate over the decades to try to achieve bipartisanship in legislation and presidential appointments.

The filibuster in one form or another dates back to the 19th century but assumed its current form in the 1970s.

While Democrats opposed the rule change and accused Republicans of a power grab, it was the Democrats who first resorted to the nuclear option when they controlled the Senate in 2013. They barred filibusters for executive branch nominees and federal judges aside from Supreme Court justices but still allowed it for Supreme Court nominees and legislation. The Republican-backed rule change maintains the ability to filibuster legislation.

In the past, the nuclear option has been averted when moderates in the two parties compromised to avoid a showdown, but the ferocious partisanship in Washington made that impossible.

The court's ideological leaning could help determine the outcome of cases involving the death penalty, abortion, gun control, environmental regulations, transgender rights, voting rights, immigration, religious liberty, presidential powers and more.

Experts said eliminating the filibuster for Supreme Court appointments could make it more likely that presidents, with little incentive to choose centrist justices who could attract support from the other party, will pick ideologically extreme nominees in the future.

Ending the filibuster would make it easier for future Supreme Court nominees to be confirmed when the president and Senate leadership belong to the same party.

Ahead of the vote, about 20 protesters with a group called Democracy Spring held a sit-in demonstration at one of the Senate's office buildings, chanting, "Stop Gorsuch" as police officers surrounded and then arrested some of them.

(Reporting by Will Dunham)