Republicans said Democrats were trying to sabotage Judge Kavanaugh’s nomination, or at least delay it long enough in hopes of payback.

“Clearly, the ultimate goal of Senate Democrats is to avenge their perceived injustice of Merrick Garland,” said Josh Holmes, a former chief of staff to Mr. McConnell. “The only way to accomplish that is to burn the clock on this nomination by any means necessary in hopes that they win the majority in the Senate and deny President Trump an appointment for the next two years.”

Some Democrats have openly suggested that they would try to follow the precedent set by Mr. McConnell and leave the current vacancy unfilled if they take charge. A Democratic Senate leader and a Democratic Judiciary Committee chairman could presumably prevent a nominee from even coming to a vote, just as Mr. McConnell did.

“I think we’ve had those kinds of vacancies before, and we certainly had over a one-year vacancy with Merrick Garland,” Senator Mazie K. Hirono, Democrat of Hawaii and a member of the Senate Judiciary Committee, told Politico Magazine’s “Off Message” podcast last week. “So the world does not come to an end because we don’t fill all of the nominees.”

Of course, just as Mr. McConnell would have to explain a change in position, Democrats would be pressed to justify a prolonged vacancy given their “We Need Nine” slogan, as they named an advocacy group that pushed for Judge Garland’s confirmation two years ago.

But nothing in the Constitution requires that the Supreme Court have nine justices; the number of seats is set by law and fluctuated in the early decades of the republic. In what was seen as a bid to thwart President Andrew Johnson from making appointments in the mid-1860s, Congress reduced the number of justices to seven from 10 and then later increased it to nine in 1869 after Johnson left office, the last time it has changed.

A two-year vacancy without changing the law, however, would go further than any Congress has gone in decades to prevent a president from using his power to appoint justices. Some Democrats dismissed such talk as Republican spin to compel wavering members like Senator Susan Collins of Maine to stand by Judge Kavanaugh for fear of what would happen if they reject him.