When an ailing Senator Dianne Feinstein asked in April to be temporarily replaced on the Judiciary Committee so Democrats could continue the panel’s work without her while she recuperated from shingles at home in California, Republicans balked, blocking the substitution. Ms. Feinstein was forced to return to Washington well before many close to her believed she was ready so her party could continue advancing President Biden’s judicial nominees.

Since then, a lingering question has hung over the Senate even as the 90-year-old Ms. Feinstein has refused to consider resigning before the end of her term in 2025: Could or would Republicans block Democrats from replacing her on the committee if she did step aside, a departure that would open the door for an appointee chosen by the state’s Democratic governor to finish her term?

No definitive answer has emerged — and the proposition may never be tested since Ms. Feinstein has said she is not going anywhere. But the question has major implications both for the Senate itself and for California politics.

Should Ms. Feinstein resign early, California’s governor would appoint a temporary senator who might then have a leg up in the hotly contested Democratic race to succeed her in January 2025. Gov. Gavin Newsom has said he would name a Black woman, which could work to the disadvantage of other candidates in the race. The idea that an early departure by Ms. Feinstein might imperil Mr. Biden’s judicial nominees has been seen as yet another reason for her to stay.

Still, senators in both parties suggest that that is unlikely, and that Republicans might relent and allow an actual vacancy on the Judiciary Committee, as opposed to a temporary opening, to be filled.

A chief reason is that the Senate is an institution heavily bound by precedent — and a deep-seated reflex to do unto others as they did unto you. Blocking a committee replacement for a lawmaker who has been forced to leave the Senate before her term is over is the type of decision that could come back to haunt Republicans, considering they have older members of their own who might find themselves in such circumstances.

Democrats would then have their opportunity to refuse someone a seat, and they would doubtless take advantage of it if they saw it as justified retaliation. As in other circumstances, the tit-for-tat could quickly escalate into a brawl that would be costly to both parties.

Members on both sides of the aisle say the difference between vacating a seat temporarily and replacing a retiring member is significant, and they believe that Democrats would be allowed to retain their majority on the influential panel.

“I can’t imagine that happening,” Senator John Cornyn of Texas, a senior Republican member of the Judiciary Committee, said about the prospect of preventing Democrats from seating a new member on the panel if Ms. Feinstein left. “I think that would be unsustainable.”

He added that the speculation on the matter “has to do with California politics and who wants to run to replace her.”

In an interview this year on CNN, Senator Lindsey Graham of South Carolina, the senior Republican on the committee who formally objected to a temporary fill-in for Ms. Feinstein, also said he “would be in the camp of replacing the person” if Ms. Feinstein or another senator opened a slot by giving up a seat early.

Not everyone is convinced. Hillary Clinton, herself a former member of the Senate, helped reignite the issue last month when she said in an interview with Time magazine that she believed Republicans would block a replacement to thwart a push on judges.

“If we’re going to get judges confirmed, which is one of the most important continuing obligations that we have, then we cannot afford to have her seat vacant,” Mrs. Clinton said.

And Senator Sheldon Whitehouse of Rhode Island, a senior Democrat on the committee, fanned the flames again this month, saying on Twitter: “The fact is simple: if Senator Feinstein resigns, Mitch McConnell gets to decide whether Democrats have a Senate Judiciary majority.”

The suspicion that Republicans could impede a replacement is driven not only by the fact that they blocked a temporary substitute, but also by the reality that Senator Mitch McConnell, Republican of Kentucky and the minority leader, is hyper-focused on the makeup of the federal judiciary. He went so far as to block Merrick B. Garland, who is now the attorney general, from getting a hearing on his 2016 nomination to the Supreme Court by President Barack Obama for nearly a year, a position that many saw as far outside Senate norms.

To Democrats, it is not much of a leap to think that Mr. McConnell would be more than willing to tie up the Judiciary Committee to slow down confirmation of Mr. Biden’s judicial picks if the opportunity arose.

Mr. McConnell declined to answer questions on the topic, with his staff saying that it was a hypothetical and that Ms. Feinstein remained in the Senate and voting. But the general sense among Republicans is that Mr. McConnell recognizes that it would break with Senate convention to block Democrats from filling a committee seat if a member left.

Some Democrats also think it is unlikely that Republicans would go that far.

“I may be naïve,” said Senator Richard Blumenthal, Democrat of Connecticut and a member of the Judiciary Committee, “but I believe they would allow the Judiciary seat to be filled if it was a resignation as opposed to just an absence. I really think a resignation could lead to replacement very quickly.”

“I don’t think you can dismiss the idea of a resignation simply because of the fear that Republicans have been destructive,” he added. “In the long run, it would be self-destructive because they are going to encounter this problem as well.”

Senator Chuck Schumer, Democrat of New York and the majority leader, said he was not sure what Republicans would do. But given their opposition to Mr. Biden’s judicial nominees, he suggested they would most likely make it difficult just to impede more confirmations.

“It’s logical they would pull out all the stops,” he said. “They might not stop it, but they could certainly slow it down.”

For her part, Ms. Feinstein has made it clear on repeated occasions that she does not intend to leave before the end of her term, making the whole question moot.

Senator Richard J. Durbin, Democrat of Illinois and the chairman of the Judiciary Committee, also said he did not know what Republicans would do. But he does know what he hopes Ms. Feinstein will do as he tries to keep advancing judges.

“I would hope that she continues to show up when needed,” he said. “And she’s been very good at that the last several weeks.”

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