WASHINGTON — President Trump may have lost the spending fight, but he can celebrate at least one significant congressional accomplishment this week: He single-handedly saved the Senatefilibuster.

By demanding in two Twitter posts on T

“There is an overwhelming majority on a bipartisan basis not interested in changing the way the Senate operates on the legislative calendar,” Senator Mitch McConnell, Republican of Kentucky and the majority leader, declared, sounding like he meant it. “And that will not happen.”

Mr. McConnell and his colleagues say that eliminating the supermajority threshold on legislation would change the very nature of the chamber, diminish American democracy and make the dignified Senate too much like the unruly House.

All that might be true. But there are other, less obvious reasons for Republicans to want to hold fast to the filibuster given the current state of play in Washington. As thebipartisan budget agreement on Sunday showed, the filibuster also provides a check on Mr. Trump by giving Democrats a voice in legislation.

Majority Republicans then have a ready excuse for not taking actions pushed by the White House that senators of both parties might deem unwise: The Democrats will not allow it. The result in the case of the spending bill was a compromise that denied the White Housemoney for a border wall while satisfying imperatives of both parties in Congress.

At the same time, Republicans privately acknowledge that eliminating the 60-vote requirement would subject them to extreme pressure from the right to unilaterally pursue an array of conservative initiatives that could ultimately cause a voter revolt. The filibuster is their political safety valve.

More important, Republicans say the filibuster has been more helpful to them in preventing Democratic legislation over the years than it has been to Democrats in thwarting Republicans. They fear that eliminating the filibuster would be catastrophic for Republicans if Democrats regain power.

“If you believe in limited government, the filibuster has been your friend far more than your foe,” said Senator Jeff Flake, Republican of Arizona, who, like many other senators, was adamant that he would not support a weakening of the tactic.

Until Mr. Trump’s latest outburst, the fate of the legislative filibuster was far from assured.

Harry Reid, Democrat of Nevada and the former majority leader, predicted last year that Democrats would try to jettison the filibuster if Hillary Clinton won the presidency and Democrats won the Senate, only to see Republicans continue the stalling tactics they employed so successfully against President Barack Obama. And Democrats had already shown a willingness to make changes with their 2013 maneuver to eliminate the 60-vote threshold for most judicial and executive branch nominees.

Then Mr. Trump’s victory completely turned the tables, with the Republicans who now controlled Congress and the White House ruminating about gutting the filibuster if Democrats stood in the way of the Trump agenda. Republicans in the House and conservative groups cheered on the idea, saying it could finally break a logjam in conservative legislation.

Early in Mr. Trump’s tenure, some Senate Republicans said a change in the filibuster rules on legislation remained a possibility if Democrats refused to give any ground to the unified Republican government. But the shaky start of the Trump administration and the ensuing chaos seem to have left Republicans on Capitol Hill less inclined to take big political risks for the president.

The certainty of the sentiment against a change was striking.

“He can say what he wants,” Mr. Flake said of Mr. Trump. “This is separation of powers. We appropriate.”

Mr. McConnell, who considers himself an ardent Senate institutionalist, was always reluctant to endorse such a radical departure from Senate tradition, and he said so repeatedly. He did, however,engineer a change that allowed Republicans to break a filibuster against Neil M. Gorsuch with a simple majority vote and put him on the Supreme Courtlast month.

But that outcome seemed less an affront to the Senate culture than payback for, and a natural progression from, the Democrats’ 2013 decision on nominations. It settled that score and made senators less interested in going further, drawing them back from the brink of more changes.

Sixty-one senators signed a letter circulated by Senators Chris Coons, Democrat of Delaware, and Susan Collins, Republican of Maine, backing the 60-vote threshold on legislation. The negative response to Mr. Trump’s morning Twitter posts — “Either elect more Republican Senators in 2018 or change the rules now to 51%. Our country needs a good ‘shutdown’ in September to fix mess!” he wrote, saying the budget compromise was only necessary because of the filibuster — appeared to seal the deal.

“This isn’t a partisan issue,” Mr. Coons said Tuesday. “Republicans and Democrats both recognize that control of the Senate will inevitably change every few years, and members of both parties value the minority party’s right to have a say in legislation.”

And, of course, it is the clout of the minority and the mere possibility of the filibuster that empowers each individual senator far beyond an individual lawmaker in the House.

The simple threat of objecting to legislation can bring them attention from their leaders, as well as the news media, and make them players in the Washington drama. To senators, that might be the best reason of all for defying Mr. Trump and holding on to their precious filibuster.