WASHINGTON — President Biden’s decision to endorse changing the Senate’s rules to pass voting rights protections was a long time coming. Perhaps — in the view of his most disaffected supporters — too long.

A self-proclaimed institutionalist who spent more than three decades abiding by those rules as a senator, Mr. Biden repeatedly defended the often-arcane procedures of the Senate, even as Republicans used them to block his agenda and he came under increasing pressure from liberal activists in his party to rethink his position.

Those rules, he said with admiration more than a decade ago, were about “compromise and moderation,” a core part of his political identity. To support changing them would be to admit that the principles he so cherished had withered in a city now consumed by partisan rancor.

On Tuesday, he made that admission.

“The threat to our democracy is so great that we must find a way to pass these voting rights bills,” he said in an impassioned speech in Atlanta on the grounds of Morehouse College and Clark Atlanta University. “Debate them. Vote. Let the majority prevail. And if that bare minimum is blocked, we have no option but to change the Senate rules, including getting rid of the filibuster.”

Mr. Biden said that he had been “having these quiet conversations with members of Congress for the last two months” in the hopes of reaching the kinds of negotiated agreements that he pursued as a senator.

“I’m tired of being quiet,” he said.

It is far from clear that Mr. Biden’s words will succeed in convincing the most prominent opponent of a rule change among Senate Democrats — Joe Manchin III of West Virginia — to help break the Republican logjam on voting rights legislation. On Tuesday, Mr. Manchin said again that he opposed “getting rid” of the filibuster, which allows the minority party to block legislation that fails to garner 60 votes.

Some of Mr. Biden’s closest allies said they remained deeply frustrated by the president’s willingness to lead from behind on the issue of voting rights.

“We had hoped he would have used his bully pulpit a long time ago for voting rights and we wouldn’t be at this critical junction,” said Helen Butler, a Black Democrat who was removed from a local election board in Morgan County, Ga., after a state law gave Republicans more power over such appointments.

“This is about retaining America and, as he put it, the soul of America,” she said.

Democrats who are trying to prevent Republicans from blocking voting rights legislation said they were pleased that Mr. Biden had finally come around. And they are hopeful — though sanguine — that his voice may help to convince a handful of senators to back a change in the filibuster rules in the days ahead.

Senator Tim Kaine, a Virginia Democrat who has been leading talks to amend the rules, said Mr. Biden came into office with a “particular obligation on his shoulders” — to stand up for voter rights in the wake of the violent assault on the Capitol last January as his election victory was being certified.

“When somebody who understands the Senate and loves it as much as he does says it’s time to make a change to accomplish a paramount result that the nation needs, it does have an effect,” Mr. Kaine said.

For some presidents, choosing to support a change in Senate rules to protect voting rights might also have foreshadowed a broader awakening to the realization that the Senate was no longer a place where partisanship could be put aside for the good of the country.

That is certainly the view of many in his party, who assert that far-reaching legislation like the president’s Build Back Better package and gun control proposals are doomed to falter without wholesale changes to the Senate’s rules.

In effect, they argue that today’s intense partisanship has led to an ideological stalemate that justifies burning down the house in the name of progress on many fronts.

“He should have made this much more a priority for his administration,” said Fred Wertheimer, the founder and president of Democracy 21, a group pushing for changes to the Senate rules. “But he still can make a critical contribution by speaking out and by actively and forcefully engaging in the battle.”

Charles McKinney, the Neville Frierson Bryan Chair of Africana Studies at Rhodes College in Memphis, said Mr. Biden’s address on Tuesday must not be the end of his efforts to make progress.

“OK, you give your little speech, say the things you need to say in Georgia,” Mr. McKinney said, describing his message to the president on Tuesday. “And then you need to be making your way back to D.C.”

Mr. Biden left no doubt that he has reached a breaking point when it comes to voting rights, lashing out at the holdouts in the Senate and comparing them to some of the country’s most infamous racists. In doing so, he made no distinction between the Senate’s Republicans and a handful of Democrats who are standing in the way of the legislation.

“Do you want to be on the side of Dr. King or George Wallace?” he declared, prompting some gasps from supporters in the audience. “Do you want to be on the side of John Lewis or Bull Connor? Do you want to be the side of Abraham Lincoln or Jefferson Davis?”

But for Mr. Biden, the slow-moving evolution from protector of the Senate rules to a president open to getting rid of the filibuster to advance voting rights legislation hardly appears to be part of a wholesale transformation in his approach to governing in the modern era.

Even as White House officials previewed Mr. Biden’s remarks on Monday night, they went out of their way to insist that he remained “an institutionalist,” an acknowledgment that his willingness to change has its limits.

Mr. Kaine said the president viewed the need to protect voting rights as a special responsibility, separate from other parts of his policy agenda.

“He probably wouldn’t be leaning into Senate rules, reform proposals on any other issue, even issues that he thinks are very, very important,” Mr. Kaine said. “I can’t imagine him probably, you know, making recommendations to the Senate about what we do with rules on any topic other than this.”

Mr. Biden’s own words over the years support that conclusion.

In July 2020, as a candidate for president, Mr. Biden hinted that his longstanding support for the Senate’s filibuster rules might have weakened a bit. Asked whether he supported eliminating the filibuster, Mr. Biden said he was open to the possibility.

“It’s on how obstreperous they become,” he said of Republicans. “But I think you’re going to just have to take a look at it.”

As pressure to pass voting rights legislation increased, Mr. Biden was still hesitant. He said last month he would support changing Senate rules to pass voting rights bills, but noted, “I don’t think we may have to go that far.”

Mr. Biden understands the political dangers of moving slowly amid significant shifts in opinion among supporters and the broader public. In 2012, as vice president, he watched as President Barack Obama was criticized by members of the gay rights movement for taking years to “evolve” on his support for gay marriage.

In that instance, Mr. Biden was ahead of Mr. Obama, and he earned plaudits from activists who became longtime supporters.

As he left the White House to travel to Georgia on Tuesday, the president was asked what he risked by fighting for voting rights legislation, a reference to the political dangers of promising more than he can deliver.

“I risk not saying what I believe. That’s what I risk,” Mr. Biden said. “This is one of those defining moments. It really is. People are going to be judged, where were they before and where were they after the vote. History is going to judge this.”