WASHINGTON - The US Senate on Tuesday approved a long-awaited infrastructure bill, after months of strenuous negotiations between Democrats and Republicans.

In a rare stroke of bipartisanship, more than a dozen Republicans joined Democrats to back the legislation. The final vote was 69-30, comfortably surpassing the 60-vote threshold required for most legislation in the 100-seat upper chamber.

Senate Majority Leader Chuck Schumer said on the Senate floor before the vote that this is a "decades overdue" step to revitalize the country's crumbling infrastructure, calling it the "most robust injection of funds" into infrastructure in decades.

In late March, US President Joe Biden unveiled a roughly 2-trillion-dollar infrastructure plan, but it was criticized by Republicans, who argued it's not targeted on infrastructure and costs too much.

After months of negotiations, a bipartisan group of senators reached an agreement on a 1.2-trillion-dollar infrastructure bill, which includes 550 billion dollars in new spending on infrastructure projects, such as roads, bridges, passenger rails, drinking water and waste water systems. The rest of the package involves previously approved spending.

"While we are pleased Republicans and Democrats have come together to move forward important public investments, this legislation falls far short of its promise to be fully paid for," Maya MacGuineas, president of the Committee for a Responsible Federal Budget, a US watchdog group, said in a statement Tuesday.

The group estimates that the bill will add about 400 billion dollars to deficits over the next decade.

MacGuineas urged the House of Representatives to improve the legislation, by supplementing the current pay-fors with additional revenue, spending reforms, and user fees, for the benefit of the nation's long-term fiscal and economic outlook.

Along with the infrastructure bill, Schumer and other Democratic leaders have been seeking to advance a separate 3.5-trillion-dollar spending plan using the budget reconciliation process, which only requires a simple majority to pass a legislation, and would allow Democrats to enact most of President Joe Biden's social-spending agenda without Republican support.

Immediately after the final vote on the bipartisan infrastructure bill, the Senate began to debate the 3.5-trillion-dollar budget plan, which focuses on childcare, education, health care and climate policy. The text of the plan was released by Democrats on Monday.

Schumer, the top Democrat in the Senate, said the infrastructure bill - despite its significance - doesn't address other challenges the country is facing, noting that Democrats believe "we need to do much more."

The Democratic budget will lower costs for Americans, cut taxes for American families, create millions of jobs while tackling the climate crisis, and "it's paid for by the wealthy and corporations paying their fair share," Schumer said.

Republicans, however, repeatedly lashed out at the massive spending plan, calling it the "socialist fantasy" for the far left, and warning that the tax hike would hurt US businesses and families, and the high price tag would exacerbate inflation and push up the already ballooning debt.

"Senate Democrats are about to take their first step toward yet another reckless, partisan taxing and spending spree. It will push costs even higher for families. It will shatter President Biden's promise of no middle-class tax hikes," Senate Minority Leader Mitch McConnell said on Twitter.

With the Senate split 50-50, Democrats must keep moderates -- who could oppose elements of the agenda -- on their side, allowing Vice President Kamala Harris to cast the deciding vote.