The Senate on Dec. 2 passed a Republican bill overhauling the tax code. The bill passed by a 51-49 vote. (Bastien Inzaurralde/The Washington Post)

The Republican tax overhaul that squeaked through the Senate early Saturday morning would reach deep into the nation’s health-care system, with a clear dagger to a core aspect of the Affordable Care Act and broader ripple effects that could threaten other programs over time.

The measure would abolish the government’s enforcement of the ACA requirement that most Americans carry insurance coverage. It would not end the individual mandate itself but would eliminate tax penalties for flouting that requirement. The result could cause an extra 13 million people to become uninsured and drive up insurance premiums in marketplaces created under the law, according to an estimate by Congress’s nonpartisan budget analysts.

Yet downstream effects of the bill that have drawn less attention could potentially damage the health care and well-being of far more people.

The Senate plan would increase the federal deficit starting in the current fiscal year and — unless lawmakers intervene — would unleash a budgetary sequence of events cutting billions of dollars from Medicare and public health services. The reductions would flow from a “pay as you go” law that basically requires offsets to increases in federal spending.

At the same time, the frenzied negotiations to line up support for the huge legislation within the Senate’s slender majority improved the prospects for temporarily reviving payments to ACA insurers that President Trump ended this fall. In becoming the last GOP senator to announce support for the tax bill, Sen. Susan Collins (Maine) said late Friday afternoon that Senate Majority Leader Mitch McConnell (R-Ky.) had “committed to support” two separate measures by the end of the year.

One is a bipartisan plan that would restore for two years “cost-sharing reduction” payments to cover the expense of discounts that the ACA compels insurers to give lower-income customers on deductibles and other out-of-pocket costs. Trump cut off the monthly payments as of October, erroneously terming them “bailouts” to the insurance industry. The plan, forged by Sens. Lamar Alexander (R-Tenn.) and Patty Murray (D-Wash.), also would expand consumers’ ability to buy inexpensive “catastrophic” health plans through ACA marketplaces and make it easier for states to secure federal permission to carry out the law’s basic ideas in different ways.

An attempt to pass the plan faltered in the Senate earlier in the fall, but Collins said Friday that McConnell was now willing to support its passage, along with a newer plan that would give states two years of money for various “reinsurance” funds intended to help insurers blunt premium increases. Neither measure has been considered by the House.

Now that both chambers of Congress have passed versions of the biggest rewrite of tax law in decades, the differences must be negotiated. The House bill would not end penalties for Americans who fail to carry insurance, but Republicans there have been sympathetic to the idea, which was part of legislation that the House adopted this year to dismantle much of the ACA.

The likelihood of big reductions in other forms of health-care spending, triggered by the pay-as-you-go law — also known as paygo — to deter deficit increases, is less certain.

In the hours before the Senate’s final vote on the tax overhaul package, McConnell and House Speaker Paul D. Ryan (R-Wis.) sought to tamp down fears of such cuts, issuing a joint statement in which they accused Democrats of “misleading claims” and promised to “work to ensure these spending cuts are prevented.”

The bill itself does not avert them, however. Separate action would be required later and — unlike the parliamentary maneuvers used to adopt the tax plan with only GOP votes — would require support from some Democrats. Republican leaders predict that Democrats would cooperate rather than bear blame for harming health-care funding.

The leaders’ joint statement has its skeptics. “We are aware they say they will waive the paygo, but we have little comfort that they can do this,” said Georges S. Benjamin, executive director of the American Public Health Association. “Why did they not write the bill to address this in the first place?”

As a final vote on the GOP tax bill neared, Senate Democrats accused Republicans of advancing their plan in secrecy. (Bastien Inzaurralde/The Washington Post)

The cuts, if they happen, would decrease federal spending on Medicare by 4 percent — amounting to about $25 billion next year, the Congressional Budget Office forecast. Because paygo rules do not allow Medicare benefits to be touched, the funding loss would be spread among payments to doctors, hospitals and others that provide care to the program’s 56 million older and disabled Americans.

Those rules focus only on the mandatory spending within the federal budget and would leave untouched some health-care programs that provide help to low-income Americans, such as Medicaid and the Children’s Health Insurance Program. But it could eliminate nearly $1 billion a year for a Prevention and Public Health Fund, created under the ACA, that now represents 12 percent of the Centers for Disease Control and Prevention’s budget.

If that fund disappears, “people are going to be sicker,” Benjamin warned, with fewer low-income Americans likely to get tested for breast or colon cancer, and public health workers less able to control outbreaks of contagious infections.

Even if the paygo cuts are averted, advocates for vulnerable groups of Americans fear that the sheer magnitude of the bill’s deficit increase — $1.5 trillion in the coming decade — would give conservatives in Congress reason to shrink social safety programs that they have long hoped to target.

“That is ultimately the most troubling part,” said David Certner, legislative counsel for AARP. “We create these large deficits, and that will put pressure for cuts to Medicare, Medicaid. . . . Everything will be on the table.”

Correction: An earlier version of this story contained a typo that misstated the potential $25 billion cut to Medicare next year.