President Biden struck an infrastructure deal on Thursday with a bipartisan group of senators, signing on to their plan to provide about $579 billion in new investments in roads, broadband internet, electric utilities and other projects in hopes of moving a crucial piece of his economic agenda through Congress.

“We have a deal,” Mr. Biden said outside the White House, standing beside a group of Republicans and Democrats after a meeting in the Oval Office where they outlined their proposal. “I think it’s really important we’ve all agreed that none of us got all that we wanted.”

Mr. Biden’s endorsement marked a breakthrough in his efforts to forge an infrastructure compromise, but it was far from a guarantee that the package would be enacted. Both the president and top Democrats say the plan, which constitutes a fraction of the $4 trillion economic proposal Mr. Biden has put forth, can only move together with a much larger package of spending and tax increases that Democrats are planning to try to push through Congress unilaterally, over the opposition of Republicans.

“If this is the only thing that comes to me, I’m not signing it,” Mr. Biden said during remarks in the East Room of the White House. “It’s in tandem.”

Still, he signaled optimism about the success of the compromise, calling it a major win for his economic agenda, for America’s competitive stance against China and for democracy itself.

“This agreement signals to the world that we can function, deliver and do significant things,” he said, standing with Vice President Kamala Harris.

Mr. Biden noted that the deal includes about two-thirds of the funding that he had called for in several parts of his American Jobs Plan, in areas like clean power and environmental resilience. He also took on liberals in his party who had criticized the negotiations, casting the outcome as both a sign of what is still possible in an increasingly polarized Washington and a first step in a process that could also include a larger budget reconciliation bill that would likely pass with only Democratic votes.

“It’s hard,” Mr. Biden said of bipartisan compromise, “but it’s necessary, and it can get done.”

It’s not clear, though, that the bipartisan plan — a product of five Republicans and five Democrats — will muster the support of at least 60 senators to overcome any filibuster. And the two-track strategy promises to be a heavy lift for Democrats in a Congress where they have only the thinnest of majorities, and moderates and progressives have very different priorities.

Still, if it succeeds, the bipartisan plan would, for the first time since President Barack Obama’s 2009 economic rescue plan, pump significant federal investments into the nation’s crumbling infrastructure — not only roads, bridges, and transit, but broadband, waterways and coastlines eroding as the planet warms.

Under the plan, $312 billion would go to transportation projects, $65 billion to broadband and $55 billion to waterways. A large sum, $47 billion, is earmarked for “resilience” — a down payment on Mr. Biden’s promise to deal with the impact of climate change.

Senator Mark Warner, Democrat of Virginia, said the package would pump around $40 billion into Internal Revenue Service enforcement to produce a net gain in tax revenues of $100 billion. A separate infrastructure finance program would leverage $20 billion in federal money to produce $180 billion in private financing on infrastructure construction.

On Thursday, Mr. Biden and the centrist senators at the White House cheered their compromise. The president, who spent more than three decades in the Senate and has staked his success on his reputation as a dealmaker, said the agreement “reminds me of the days we used to get an awful lot done up in the United States Congress.”

“This does represent a historic investment in our nation’s infrastructure,” Senator Kyrsten Sinema, Democrat of Arizona, who helped spearhead the talks that led to the agreement, said at the White House.

But it would leave large swaths of the president’s economic proposals — including much of his proposed spending to combat climate change, along with investments in child care, education and other social programs — for a potential future bill that Democrats would try to pass without any Republican votes using a procedural mechanism known as reconciliation.

On Capitol Hill, Democrats signaled openness to accepting the initial details of the agreement, provided that their moderate colleagues accept a second, much larger reconciliation package.

“There ain’t no infrastructure bill without the reconciliation bill,” Ms. Pelosi told reporters on Thursday, repeating a comment she had made privately on a call with House Democratic leaders, according to two officials familiar with it.