WASHINGTON (Reuters) - Democratic opposition to President Donald Trump’s U.S. Supreme Court nominee grew on Monday while the White House demanded a “fair, up-or-down vote” in the Senate on confirming Neil Gorsuch to the lifetime post.

Supreme Court nominee judge Neil Gorsuch responds to a question as he testifies during the third day of his confirmation hearing. REUTERS/Jim Bourg

Four more Democratic senators added their support to a growing effort to block a confirmation vote through the use of a filibuster. That procedural hurdle requires 60 votes in the 100-seat Senate to allow a confirmation vote by a simple majority. Republicans control the Senate 52-48.

There are now 20 senators who have backed Democratic leader Chuck Schumer’s filibuster call - up from 16 on Friday.

Senator Joe Manchin, however, announced on Monday that he opposes a filibuster, according to an aide, and Senator Heidi Heitkamp also indicated she would oppose it.

“It’s my duty to fully consider any Supreme Court nominee, regardless of which party is in the White House,” she said in a statement.

Democratic opposition to Gorsuch could prompt a Senate showdown over the confirmation of the conservative appeals court judge from Colorado, but Republicans could change the Senate’s rules to disallow filibusters against Supreme Court nominees.

Trump is seeking to avoid another setback in Congress after major healthcare legislation he supported was pulled from the House of Representatives floor amid Republican opposition on Friday.

The confirmation of Gorsuch, 49, to replace Antonin Scalia, who died in February 2016, would restore the nine-seat court’s conservative majority, a major campaign promise for Trump.

The latest Senate Democrats to join the fight include Bill Nelson, who said there are “real concerns” that Gorsuch would be hostile to voting rights and support businesses over workers and consumers.

The other three Democratic senators to have newly announced their opposition were Judiciary Committee members Mazie Hirono, Al Franken and Jack Reed.

Manchin and Nelson are among 10 Democrats who conservative activists had hoped would join Republicans in preventing any attempt to block a Gorsuch vote. They are all up for re-election in 2018 in states Trump won in the 2016 presidential election.

Democrat Patrick Leahy of Vermont, also a Judiciary Committee member, said he generally opposes filibusters but added that Gorsuch would face one if he does not adequately answer written questions.

White House spokesman Sean Spicer said any attempted filibuster would be “nothing short of obstructionism.”

The committee is due to vote next Monday on sending the nomination to the full Senate.