Despite what the average voter probably thinks, presidential candidates keep the overwhelming majority of the promises they make. And most of the ones they don't keep aren't because they were just lying, but because circumstances changed or they tried to keep the promise and failed. But that's in the big, broad strokes, while the details are another matter. It's easy to put out a plan for, say, tax reform, but even if you achieve tax reform, it's Congress that has to pass it, and they will inevitably shape it to their own ends. This happened to a degree with President Obama's health care reform: it largely resembles what he proposed during the 2008 campaign, but not entirely. He had said he wanted a public option, for instance, but eventually jettisoned that, and had rejected an individual mandate, but eventually embraced it as unavoidable.
Which brings us to Mitt Romney's health care plan. In its details, it's quite horrifying. Jonathan Cohn has done us the service of giving it a close read, and explains: "He wants to scale back health insurance, so that it reaches less people and provides less protection from medical bills. In theory, this transformation will unleash market forces that restrain the cost of medical care. In practice, it will cause serious hardship, by exposing tens of millions of Americans to crushing medical bills or forcing some of them to go without necessary, even life-saving care." Estimates are that under Romney's plan—which repeals the Affordable Care Act, makes Medicaid a block grant (leading almost inevitably to fewer people getting covered), eliminates the tax advantage for employer-sponsored coverage (leading to more employers dropping coverage) and turns Medicare into a voucher, as many as 58 million fewer Americans could have health insurance than will once the ACA fully takes effect. Wow.
So the question is, is Mitt Romney really going to do this? I'm guessing the answer is no, and here's why. If he becomes president, he'll confront health care under one of two scenarios. The first is one in which the Supreme Court has upheld the ACA. In that case, conservatives are still mad, and will want to repeal it. But as long as there are more than 40 Democrats in the Senate to mount a filibuster, they won't let repeal happen. So faced with the inability to achieve great big things on health care, Romney will probably settle for some smaller bills, probably including malpractice reform. One year into his presidency, the ACA will take full effect, and at that point, implementing his plan would mean not just preventing people who don't have insurance from getting it, but actually tossing people who have insurance off their plans. Which just isn't going to happen.
The second scenario is that the Supreme Court overturns the ACA, in which case they will have largely done Romney's job for him. The elements of his plan that don't relate to the ACA—block granting Medicaid, ending the tax exemption for employer benefits—will still run into unified opposition from Democrats, and as far as congressional Republicans will be concerned, the battle over health care will be over, and they'll move on to other things.
In any discussion of health care, it's important to remember that Republicans don't really care about the issue, except insofar as it's a bludgeon they can use to beat Democrats with. They just don't. They care about taxes, and regulation, and defense, and many other things, but they're happy not to worry about health care unless they have to. So chances are that whatever the Supreme Court decides, big, dramatic changes to the health care system during a Romney presidency are going to be talked about briefly, then put on the back burner permanently.