WASHINGTON (Reuters) - A third of Democratic senators have so far announced they will vote against confirming U.S. President Donald Trump’s Supreme Court nominee, adding to a chorus of opposition from the left, but leaving questions over whether there will be a concerted effort to block a vote in the Senate. To date, 16 of the 48 Democratic senators have publicly backed their leader, Chuck Schumer, who said on Thursday he opposes confirming appeals court judge Neil Gorsuch. Others have yet to announce their intentions.  Gorsuch is headed toward a Senate Judiciary Committee vote, likely on April 3, on his nomination to the high court after a marathon four-day confirmation hearing this week.  After that, it is not yet clear how much of a fight Democrats will put up when the nomination is due for a final vote in the 100-member Senate, where there are 52 Republicans.   The Democratic opposition to Gorsuch could prompt a showdown in the Senate and delay the judge’s confirmation but ultimately the Republicans are likely to win that fight and avoid another setback in Congress for Trump who suffered a blow on Friday when lawmakers pulled a major healthcare bill. Senate rules enable Democrats to insist on 60 votes to overcome a procedural move called a filibuster to allow a final up-or-down vote on confirming Gorsuch, 49, to the lifetime job on the nation's highest court. (GRAPHIC - Confirming Gorsuch: How it works tmsnrt.rs/2nANgEj)  But the Senate’s Republican leadership could adopt a rule change allowing a vote that would only require a simple majority of the chamber. If eventually confirmed, Gorsuch would restore a conservative majority on the nine-seat high court. For Democrats, putting up a fight would at least frustrate Trump and placate liberal activists sore about the Republican-led Senate’s refusal to vote on President Barack Obama’s nominee to the same open seat on the court last year.  If eventually confirmed, Gorsuch would restore a conservative majority on the nine-seat high court.  Conservative activists were targeting ten Democrats running for reelection in 2018 in states Trump won in the presidential election as possible “yes” votes for Gorsuch among Democrats.  Of that number, Sherrod Brown of Ohio, Bob Casey of Pennsylvania and Tammy Baldwin of Wisconsin have already announced their opposition to the nomination. The other seven, including Joe Manchin of West Virginia, have yet to announce their plans. Another Democratic senator under pressure from both sides on Gorsuch is Michael Bennet, who represents Gorsuch’s home state of Colorado. Bennet has yet to announce his position. Republicans are also hopeful that some Democratic members of the judiciary committee, including Chris Coons of Delaware and Amy Klobuchar of Minnesota, might be wary of blocking a vote on the nominee. Their spokesmen did not respond to requests seeking comment. Don Stewart, a spokesman for Republican Senate Majority Leader Mitch McConnell said it would be an “unprecedented, even nuclear step” to require a 60-vote threshold before the Senate can vote on the nominee. “There has never, in the history of the Republic, ever been a successful partisan filibuster of a Supreme Court nominee,” he added. The last attempt to filibuster a nominee was a Democratic bid to block a vote on Justice Samuel Alito, who was nominated by President George W. Bush in 2005. Democrats failed to gather enough support and the Senate voted 72-25 to proceed to an up-or-down vote. Alito was eventually confirmed by 58-42.  Sarah Binder, a political scientist at George Washington University, said Democrats may be able to gather enough support to block a vote because so few of them are moderates from Republican-leaning states, meaning they may feel they have little to lose. If Trump’s popularity drops, that could embolden them further, she added.    