WASHINGTON — Before the election, it was clear that Democrats were looking at the very real prospect of scrapping the filibuster to keep Republicans from blocking the Supreme Court nominees of a President Hillary Clinton.
Now the tables are turned. It is the Republicans who will control the White House, Senate and House — the coveted Washington trifecta. And that raises a crucial question: If Democrats in the even more narrowly divided Senate embrace the filibuster to block what could be a flood of legislation, would Republicans respond by eliminating the 60-vote threshold in order to push their priorities through on simple majority votes?
In the afterglow of their election success, Republicans prefer not to discuss this unpleasant possibility. They would rather rhapsodize about a sunny legislative future in which the two parties work in harmony, negating the need for all those troublesome Senate cloture votes to try to break filibusters (a tactic that they, in fact, employed very effectively to stymie President Obama).
“I think what the American people are looking for is results,” Senator Mitch McConnell of Kentucky, who will retain his majority leader title by virtue of surprising Republican victories, told reporters. “And to get results in the Senate, as all of you know, it requires some Democratic participation and cooperation.”
At the same time, Mr. McConnell said that repealing the Affordable Care Act was a “pretty high item on our agenda,” and he predicted quick action. “The sooner we can go in a different direction, the better,” he said.
Democrats are certain to oppose that. Even if some red-state Democrats up for re-election in 2018 join Republicans, the repeal effort will most likely remain short of the 60 votes now needed.
Republicans can use a special process known as reconciliation to avoid a filibuster. But that effort would take well into 2017 and would require passage of a congressional budget resolution, among other steps.
Presumably, the newly installed president, Donald J. Trump, along with eager House Republicans and impatient legions of Trump supporters, would want to see the law repealed well before then, and they could apply enough pressure to lead to the filibuster’s demise.
Another development could spur a filibuster fight even sooner. Mr. Trump is likely to move quickly to fill the Supreme Court seat that Senate Republicans have kept empty for more than eight months.
Senate Democrats say they hope that Mr. Trump follows tradition by consulting with them on his Supreme Court choice, and that he picks a nominee both parties are able to support. But they remain very unhappy about how Republicans treated Merrick B. Garland, the highly regarded federal appeals court judge whose nomination by Mr. Obama in March is now dead.
Democrats may want some payback. So if Mr. Trump picks a conservative whom Democrats find objectionable, it is possible that they will do whatever they can to impede the choice, including a filibuster.
That could lead to a retaliatory move by Republicans to eliminate the filibuster against Supreme Court nominees, the only judicial candidates exempted from a change Democrats engineered in 2013 that allowed filibusters against nominees to be broken by a simple majority vote.
Mr. McConnell would not tip his hand this week on how he would respond to such a Democratic approach. But he did not rule out the so-called nuclear option: changing rules via a procedural vote on the floor instead of the 67 votes officially required.
“I would not anticipate any particular strategy that the Democrats might employ to defeat it or what we might do in reaction to that,” he said.
Mr. McConnell is what is known on Capitol Hill as an institutionalist, a strong believer in the traditions and practices of the Senate. He was very critical of the decision by Senator Harry Reid of Nevada, the Democratic leader, who is retiring this year, to rally his party into limiting filibusters against nominees in response to Republican delaying tactics.
Mr. McConnell has said repeatedly that it is crucial to American democracy to respect the special rights of the minority party in the Senate, and that it would be a mistake to limit the filibuster, since the decision could backfire if his party fell out of power.
“I don’t think we should act as if we’re going to be in the majority forever,” he said.
It is not clear that Mr. McConnell, even if he wanted to, could round up enough votes from his own party to curtail the Senate filibuster, since some veteran Republican senators such as Susan Collins of Maine would probably be opposed.
However, if Senate Democrats were able to frustrate the unified Republican government through the filibuster, the forces behind Mr. Trump’s election and agenda — among his top priorities are major tax cuts and tough border security measures — would be likely to push Mr. McConnell and his colleagues to get rid of it.
House conservatives and other top Republicans were already agitating for an end to the filibuster last year, and that was when Mr. Obama was still certain to veto bills he opposed.
In addition, the 2018 electoral landscape is so favorable to Republicans that they are very likely to retain power for the entire four years of Mr. Trump’s term, giving them a huge opening to rewrite the federal statute book if left unimpeded.
Mr. McConnell loves the Senate rules and would be loath to change them. But he did not reverse the Democrats and restore the 60-vote threshold on most nominees when the Republicans took control in 2015. He might ultimately be persuaded to scuttle the filibuster on Supreme Court nominees and even on legislation if he felt it was justified and the time was right.