Mitt Romney, speaking just before noon today, declared that on his first day in office, “I will act to repeal Obamacare.” I think he chose his words carefully. As President, he may indeed “act” to repeal it on Day One, but I don’t believe he will actually be able to overturn the law.

If Romney were to win in November, the first matter he’d have to deal with would be the fallout from the so-called fiscal cliff of December 31st, the day when some five hundred billion dollars worth of tax increases and spending reductions take effect, which could put the economy into another recession (if it’s not already in recession by then). This moment would perhaps be Romney’s greatest chance at repeal. Because the fiscal-cliff negotiations will be an enormous fight over the size and scope of the federal government, every government policy will theoretically be open to debate—including, Romney might insist, repeal of the A.C.A.

But it’s a fantasy. The negotiations would be dead before they started if Republicans demanded repeal as a price for a Grand Bargain on taxes, spending, and entitlements. The fiscal-cliff negotiations will undoubtedly include a great deal of horse-trading that will infuriate and cheer partisans on both sides. But there is literally nothing Republicans could offer Democrats in return for repealing the Party’s greatest achievement since the Johnson Administration.

Assuming that Romney comes through this period of his transition and Presidency with a deal that settles the tax and spending issues brought about by the fiscal cliff (and the related debt-ceiling vote that will likely happen weeks later), he could then return to his domestic agenda, which, he declared today, includes repeal of the A.C.A. as the first priority. But he would immediately face a set of political circumstances similar to the ones that made health care such a difficult issue for Obama in 2009.

Absent the Senate Democratic Caucus being found to be running a crack house or chid-prostitution ring, there is no prospect whatsoever of the Republicans winning a sixty-vote, filibuster-proof majority in the Senate this year. The most likely outcome is the Democrats narrowly retaining control, though Republican control is certainly within the realm of possibility.

Assuming that Romney comes to Washington without a sixty-vote majority in the Senate, the task of repeal will be nearly insurmountable. First of all, the Congressional Budget Office, which “scores” all legislation—and which so frustrated Obama in 2009 that he refused to mention its name in White House meetings, demanding instead that aides call it “banana”—would now be the A.C.A.’s best friend. The last time the C.B.O. weighed in on the matter, it reported that repeal of the A.C.A. would cost the government almost three hundred billion dollars. Republicans dispute that, but they’d still be under pressure to explain where they would come up with that money.

The bigger problem, of course, would be in the Senate. Remember the weeks that the Senate Finance Committee spent arguing over health care? The committee would need to return for a repeat performance. If Democrats still controlled the committee, Republicans would have to somehow force it to debate repeal and find at least one Democratic vote to send repeal legislation to the full Senate. This is unlikely to happen.

But if it does, in order to become law, Romney’s repeal of the A.C.A. would face a battery of three separate tests requiring sixty Senate votes: one to bring the legislation to the floor, one to start the debate, and one to end the debate. The filibuster, the G.O.P.’s favorite parliamentary device of the Obama era, would now be the Party’s great enemy.

Many Republicans, especially in the blog and talk-radio swamps, would cry, “Use reconciliation!” Readers familiar with the congressional debates of 2009-2010 will remember that this procedure allows certain budgetary measures to pass through the Senate with a simple majority. (After Ted Kennedy died and was replaced by the Republican Scott Brown, Obama and congressional Democrats used the reconciliation process to make some final, crucial changes to the health-care law.) But reconciliation wouldn’t work here—the process can only be used for policies that have budgetary effects and a C.B.O. score. Much of the A.C.A., such as the insurance exchanges and subsidies, would fall under these categories. But a lot of it, including the hated individual mandate, does not. Repealing the exchanges and subsides without repealing the mandate and the other regulations and cost controls in the law would create a health-care Frankenstein that a President Romney would be rather nuts to support.

If the Supreme Court had gutted the law today by throwing out the mandate and the regulations and several of the other “non-scoreable” items, a President Romney with a G.O.P. Congress might have had a relatively easy time finishing the job of killing Obama’s law, even without sixty votes in the Senate. But today the Court did two things that make repeal of the A.C.A. nearly impossible now: it has given its not-inconsequential stamp of legitimacy to the law, and it has made the parliamentary path of repeal through Congress highly unlikely and probably impossible, at least in the near future.

Far-sighted conservatives always thought that their great hope for toppling Obama’s most important legislative achievement was through the courts. They were correct.

Read The New Yorker’s full coverage of the health-care decision.

Photograph by Charles Dharapak/AP Photos.