WASHINGTON — Senate Democrats on Thursday vowed to filibuster the Supreme Court nomination of Judge Neil M. Gorsuch, signaling an imminent partisan showdown over the nominee’s fate and the future of century-old rules in the chamber.

As committee hearings on Judge Gorsuch concluded on Thursday, it appeared increasingly likely that Republicans hoping to elevate President Trump’s choice for the court would resort to replacing longstanding rules with a simple majority vote on his confirmation.

While a parade of witnesses addressed the Senate Judiciary Committee, trading dueling views of Judge Gorsuch, the Senate Democratic leader, Chuck Schumer of New York, went to the Senate floor and announced that he would try to lead Democrats in blocking an up-or-down vote on Judge Gorsuch. The Senate’s “cloture” rule requires a supermajority of 60 votes to overcome such a filibuster.

“After careful deliberation I have concluded that I cannot support Judge Neil Gorsuch’s nomination to the Supreme Court,” Mr. Schumer said, citing concerns over Judge Gorsuch’s record on workers’ rights and his degree of independence, adding, “My vote will be no, and I urge my colleagues to do the same.”