Senate Republicans squeaked their sweeping tax and health policy bill over the finish line on Tuesday by the narrowest of margins, but the hard-fought legislative win came at considerable risk to their party’s political futures and fiscal legacy.

The measure, which still faces a tough road to final passage in the House, would make steep cuts to popular health and nutrition programs among other actions to help pay for roughly $4.5 trillion in tax cuts. It is projected to drive up the national debt considerably over the next 10 years despite vociferous protests to the contrary by President Trump and Republicans in Congress.

It took an extraordinary all-night session and some last-minute sweeteners for the final holdout for Republican leaders to drag the bill to passage, an effort that underscored the deep unease about the legislation. Senator Lisa Murkowski, the Alaska Republican who was a final holdout and backed it only after securing protections for her constituents from its most painful cuts, called the process “agonizing.”

Now, the G.O.P. must sell its widely criticized plan to a public that polls show is already skeptical of the legislation. And it will have to do so over the harsh condemnation of Democrats, who intend to make attacks on the measure a centerpiece of their 2026 campaign to retake the House and Senate.

The long-term fiscal consequences of the measure may be even more damaging. In the messy process of jamming the bill through the closely divided Senate, Republicans shattered a longstanding budgeting convention meant to curb Congress’s ability to enact policies that swell the deficit in the long term. With that guardrail busted, the nation’s already soaring debt is at risk of increasing exponentially over time, a dubious legacy for Republicans, who often call themselves the party of fiscal responsibility.

Some of the most lacerating criticism of the legislation came from within the Republican ranks, notably from Senator Thom Tillis, Republican of North Carolina. He had been warning his party for days that the bill was a political loser even before he abruptly announced on Sunday he would retire from the Senate next year after coming under political assault from Mr. Trump over his opposition to the bill.