WASHINGTON — Joe Biden’s pitch during the 2020 campaign to unseat President Donald J. Trump was simple: Trade in a stubborn, immovable leader for one with a proven record of taking half a loaf when a full one is out of reach.

That approach appears to have brought Mr. Biden to the precipice of victory on a $2 trillion deal that could begin to define his legacy as a successful Oval Office legislative architect, one who is reshaping government spending and doing so by the narrowest of margins in a country with deep partisan and ideological chasms.

But the bill is certain to be far smaller than what he originally proposed, and far less ambitious than he and many of his allies had hoped. It won’t make him the one who finally secured free community college for everyone. Seniors won’t get free dental, hearing and vision coverage from Medicare. And there won’t be a new system of penalties for the worst polluters.

“Look — hey, look, it’s all about compromise,” Mr. Biden said at a CNN town hall meeting on Thursday, shrugging off the doubters as he sought to close the deal with lawmakers and the public.

But accepting less and calling it a win has its limits — and consequences.

By spending the last several months pushing for an even larger and more ambitious agenda, knowing that he would most likely have to pare it back, Mr. Biden has let down some supporters who believed he could deliver on his soaring rhetoric about the need for better higher education, expanded Medicare services and bold advances in the fight against climate change.

“In order to make real progress, you have to inspire people about the importance of the work,” said Doug Elmendorf, the dean of the Harvard Kennedy School and the former director of the Congressional Budget Office. “And then any compromise is a disappointment.”

Negotiations are continuing on the package’s final framework. But after pushing for months for a $3.5 trillion bill, the president is likely to end up with a measure including less than $2 trillion in spending spread across various initiatives, including education, child care, paid leave, anti-poverty and climate change programs.

Once the spending bills are behind him, Mr. Biden still faces challenges that are not so easily solved by compromise. On Thursday, he appeared to acknowledge that reality by hinting that he was open to altering the Senate’s longstanding filibuster rules if that is what it takes to break through Republican opposition to protecting voting rights and passing other parts of the Democratic agenda.

“We’re going to have to move to the point where we fundamentally alter the filibuster,” he told the CNN anchor Anderson Cooper.

That is a dramatic concession for a politician like Mr. Biden, who embraced the often arcane rules of the Senate during the three decades he served there. Like other institutionalists in the chamber, Mr. Biden has resisted demands from liberal activists to shatter those rules, fearful of the consequences the next time Republicans are in charge.

But the Washington that Mr. Biden often reminisces about — the one in which Democrats and Republicans work together toward common goals — is largely a distant memory. If he wants to make progress on voting rights, climate change, prison reform, an immigration overhaul and more, he most likely won’t be able to lean on the same instincts that have animated most of his political life and defined the brand that helped him win the White House.

The political differences are stark: Republicans argue the president’s spending program would burden future generations with more debt and serve as a drag on the economy. They insist the voting rights legislation is intended to benefit Democrats, and they oppose many of the president’s climate policies because they say they will be bad for jobs and business.

John Podesta, who served as former President Bill Clinton’s chief of staff, said Mr. Biden “has done a pretty good job of pushing as far as he could in the style that he was a champion of.” But he said that beyond the spending bills, “it’s hard to see how he gets that same spirit of collaboration, good will, honorable compromise.” The issue of voting rights may be the clearest example in the months to come.

Just this week, Republicans used the filibuster to block an already watered-down Democratic voting rights bill for the third time since Mr. Biden took office. The takeaway? If Democrats want federal legislation to stop what they view as an assault on voting in Republican-controlled states, they will need to play hardball.

That very likely means persuading all 50 Democrats and independents in the chamber to vote for changing the filibuster rule — if he can.

“President Biden and Senate Democrats need to fulfill campaign promises and defend our democracy — there’s too much at stake,” leaders of Fix Our Senate, a group that favors eliminating the filibuster, said in a statement on Friday. “After three Republican filibusters of common sense voter protection laws, it’s time to end the filibuster and protect the right to vote for all Americans.”

Advocates for immigrants’ rights are poised to make a similar argument on behalf of fixing what most agree is a broken system. The idea of a bipartisan immigration overhaul, which Mr. Biden proposed on his first day in office, has stalled amid opposition from Republican lawmakers.

An attempt to use the spending bill to provide a path to citizenship for millions of undocumented people was blocked by special Senate rules on the budget. If Mr. Biden wants to make good on his promised immigration overhaul, it would require a separate bill, and he may have no choice but to change the filibuster rules for that issue as well.

But perhaps the biggest promise Mr. Biden made during the campaign was to be the president who would finally confront the environmental dangers facing the planet. On Thursday, he put it in the bluntest possible terms: “The existential threat to humanity is climate change.”

Mr. Biden and his party are likely to face that threat alone in the coming months and years. Most Republicans have shown little appetite for aggressive action to counter the environmental damage from cars, manufacturing and other economic activities.

And even within his own party, the president faces divisions that make it difficult to convince the rest of the world that the United States is serious about reducing the emissions that are causing global warming.

For Mr. Biden, then, the question will be: Is he willing to treat the debate of core issues like the climate, voting rights and immigration as a “break the glass” moment in which he and his Democratic allies have no choice but to change the rules, even if it means Republicans will take advantage of the chance to advance their own agenda once they return to power?

Of course, the president can’t change the filibuster by himself. Doing so would take the votes of all 50 Democratic and independent senators, along with a tiebreaking vote from Vice President Kamala Harris. And already, some of the same Democratic senators who forced him to accept less spending have said they oppose altering the filibuster.

But the president is the leader of his party, and his voice matters. If he decides it’s time to “fundamentally alter the filibuster,” as he said on Thursday, members of his party may listen.

One argument at his disposal: Changing the rules to allow more of the Democratic agenda to pass could be vital for the party’s success at the polls.

Strategists say enthusiasm among core Democratic voters is critical to defeating the Republican Party in the midterm elections of 2022 (and perhaps Mr. Trump, its leader, two years later). If crucial parts of the president’s coalition remain unhappy because they are disappointed in the compromise bill, that could threaten Democratic hopes to remain in power in Congress and the White House.

“The political costs of this will be large,” Mr. Elmendorf said.

Mr. Podesta, who advised Hillary Clinton during her runs for the presidency, agreed. He said it was a “big problem” if Democrats couldn’t deliver on the fundamental promises. Mr. Biden’s approval ratings have already fallen into the low-to-mid-40s.

“Particularly younger voters,” he said. “You are seeing it among independents, African American and Latino voters. They are just feeling like these guys are not delivering.”

Mr. Biden might turn the enthusiasm gap around by making progress on the rest of his agenda, including voting rights, immigration and climate change.

He vowed on Thursday that he would continue to press for parts of his agenda that were left on the cutting room floor during the debate over the spending bills. He called increasing Pell Grants for college students “a start” but he pledged to keep trying for free community college — in part to satisfy the demands of his wife, Jill Biden, who is a longtime college professor.

“I’m going to get it done,” he vowed, adding with a smile at his wife in the front row, “and if I don’t, I’ll be sleeping alone for a long time.”