Widespread defections on that basis seem highly unlikely.

And Senator Susan Collins, the moderate Republican from Maine who broke with her party and backed holding a confirmation hearing and vote on Judge Garland in 2016, said she would take the same position in 2020: Should a vacancy arise, the sitting president should get the chance to choose a nominee, and the Senate should move forward to confirm.

“My standard on the nomination of Supreme Court nominees remains the same,” she said. “As long as the president is in office, he has the constitutional right to nominate. I thought that Merrick Garland should have had a hearing and a vote. Now obviously, senators could have voted against him based on the timing. But to block the nomination from proceeding at all, I thought was wrong.”

Senator Charles E. Grassley, Republican of Iowa, oversaw the Judiciary Committee but refused to convene a hearing for Judge Garland and met with him only grudgingly. As a result, he said last year that he would not consider a nominee in 2020 if he were still chairman of the panel. But he has since left the top spot on the panel to take over the Finance Committee, sparing him the prospect of either going back on his word or infuriating Mr. Trump and his colleagues. Allies say they doubt he would take a stand against a nominee since he is no longer chairman.

Republicans say the difference between 2016 and 2020 is one of political alignment. Democrats held the White House and Republicans controlled the Senate in 2016; Republicans now control both. To Mr. McConnell and his colleagues, that shift justifies their new position. But in 2016, Republicans focused most of their argument against taking up Mr. Obama’s nominee not on party control, but on the basis of the approaching presidential election, and they would face thunderous charges of hypocrisy if they took up a nomination next year.

Democrats remain angry over the treatment of both Mr. Obama and Judge Garland and expect Republicans to move aggressively if the chance arose for Mr. Trump to place a third nominee on the court. Should any nominee replace one of the four justices picked by Democratic presidents, it would cement a commanding 6-to-3 conservative majority on the Supreme Court, and the lure of that lineup would probably prove irresistible to Republicans.