WASHINGTON (Reuters) - A Democratic senator delivered a 15-1/2-hour, all-night speech denouncing President Donald Trump’s U.S. Supreme Court nominee on Wednesday, joining an effort to block Senate confirmation of Neil Gorsuch in a heated political showdown with Republicans. Senator Jeff Merkley of Oregon began his speech on the Senate floor on Tuesday evening and wrapped up mid-morning on Wednesday. The Senate was expected to vote on Thursday at 11 a.m. to try to end a Democratic procedural effort called a filibuster aimed at blocking Gorsuch’s confirmation to a lifetime post on the court. Republicans were expected to fall short of being able to halt the filibuster, but said they had the votes needed to then immediately change the Senate rules to prohibit filibusters against Supreme Court nominees. Republicans said Gorsuch would  be confirmed on Friday one way or the other. Senate confirmation of Gorsuch, 49, would reinstate the court’s conservative majority, allow Trump to leave an indelible mark on America’s highest judicial body and fulfill a top campaign promise by the Republican president. Toward the end of his marathon speech, Merkley looked weary, his suit jacket unbuttoned and his yellow tie billowing out. He stood beside an easel holding graphics that an aide would periodically adjust. “For the first time in U.S. history, a seat has been stolen from one president and delivered to another in a court-packing scheme. If that were to succeed, it would set a precedent that will haunt the court for decades to come,” Merkley said. He noted that the Republicans who control the Senate refused last year to consider former Democratic President Barack Obama’s nomination of appellate judge Merrick Garland to replace Justice Antonin Scalia, who died in February 2016, the same seat Gorsuch now has been named to fill. Later on the Senate floor, Republican Senator Cory Gardner countered that such an argument amounted to advocating that “two wrongs must make a right.” Merkley criticized Gorsuch’s legal opinions and said Gorsuch would become a conservative legal activist on the court. At one point, Merkley read a 1998 speech by one of the past giants of the Senate, Democrat Robert Byrd, decrying partisanship in Congress.  “I believe that the American people are more than tired of partisan warfare. I believe they wish for less of it from Congress, especially in the Senate, where more statesmanship and a longer view are expected,” Merkley said, quoting Byrd. The Senate has a lengthy history of long speeches, including notable ones in recent years by Senators Ted Cruz, Chris Murphy and Rand Paul. The Senate Historical Office listed Merkley’s speech at 15 hours and 26 minutes, the eighth longest in Senate history. The longest came in 1957 when segregationist Senator Strom Thurmond spoke for 24 hours and 18 minutes against legislation, later enacted, protecting black voting rights. On the second day of formal Senate debate on Gorsuch’s  nomination, various Republicans called the conservative Colorado-based federal appeals court judge “incredibly qualified,” an “intellectual heavyweight” and “always true to the law.” “Democrats are bowing to hard-left special interests that can’t get over the results of the election and thus are demanding complete Democratic opposition to everything this president touches,” Majority Leader Mitch McConnell said. 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. Republicans control the Senate 52-48. The rule change, which requires a simple majority, has been dubbed the “nuclear option,” and Trump has urged McConnell to “go nuclear.” A filibuster requires a super-majority of 60 votes in the 100-seat Senate in order to proceed to a simple majority vote on a Supreme Court nominee or legislation. The 60-vote super-majority threshold empowers the minority party to hold up the majority party and has forced the Senate over the decades to try to achieve bipartisanship on legislation and presidential appointments. Republican Senator John McCain, known as a defender of Senate traditions, offered reluctant support for the rule change, but said it likely would lead to judicial nominees “from the extremes of both left and right.” “What we are about to do at the end of this week will have tremendous consequences, and I fear that some day we will regret what we’re about to do. In fact, I’m confident we will,” McCain said on the Senate floor. The White House denied on Wednesday that Gorsuch had engaged in plagiarism after media reports accused him of copying some language and failing to cite relevant sources in his 2006 book about assisted suicide and euthanasia. “There is only one explanation for this baseless, last-second smear of Judge Gorsuch: those desperate to justify the unprecedented filibuster of a well-qualified and mainstream nominee to the Supreme Court,” White House spokesman Steven Cheung said in a statement. 