The key domestic policy fight of 2017 will be over Obamacare. If it is repealed, then the centerpiece of the Obama presidency will lie in ruins. If not, then President Obama will have been what he set out to be: a sort of Reagan of the left—a transformative president who will have profoundly changed the direction of the country.

If Donald Trump is the Republican nominee, he won't win—or even wage—a fight to repeal Obama's namesake. Trump has declared during this campaign that "single payer" heath care "works incredibly well" in Scotland. He has repeatedly indicated that, despite our nearly $20 trillion national debt, he has no real interest in tackling the entitlement spending that is bankrupting us. And when asked a few weeks ago to give his "opinion" on what "the top three functions of the United States government" are, he said one of them is "health care."

This is not a man who intends to repeal Obamacare. Sure, Trump checks the rhetorical box in saying the opposite, knowing that to do otherwise would be to commit political suicide in a Republican primary. But in addition to his general enthusiasm for government monopolies over health care abroad, disdain for entitlement reform, and belief that the provision of health care is one of the federal government's three core functions, Trump's campaign has indicated even more specifically why repeal would be a pipe dream under a Trump presidency.

First, just a few weeks ago, Trump said, "I'm not going to cut Medicare or Medicaid," which means he wouldn't roll back the huge Obamacare Medicaid expansion that John Kasich and others chose to implement. Indeed, the framework of Trump's skeletal Obamacare alternative suggests he would increase Medicaid coverage even beyond Obamacare's already bloated levels. The Congressional Budget Office projects that more than $1 trillion of the $1.9 trillion gross cost of Obamacare over the next decade will come from its Medicaid expansion ( see Table 3). So if Trump won't cut Medicaid spending, that means he wouldn't repeal more than half of Obamacare (at least in terms of dollar values).

Second, Trump has repeatedly said he wouldn't repeal Obamacare's preexisting-conditions mandate. This mandate undermines the very notion of insurance, which dates back at least to the Renaissance—namely, that one must buy insurance before the thing happens that one is insuring against. (This mandate is really two mandates in one: automatically approved insurance ("guaranteed issue") to sick people at the same price as health people ("community rating").) This mandate is a—probably the—key reason why Obamacare is causing premiums to spike.

To be sure, a conservative alternative to Obamacare should provide commonsense consumer protections—guaranteeing, for example, that someone who is insured by an employer and then shifts to individually purchased insurance couldn't be charged more for a condition he or she had covered under the prior plan; and that those who turn 18 and/or move off of their parents' insurance have a one-time buy-in period when they can't be charged more for a preexisting condition that may or may not have been covered under their parents' policy. But so long as Obamacare's preexisting-condition mandate remains in place, no conservative alternative can meaningfully bring down health costs or fix the market for individually purchased insurance that the federal government has broken.

Third, the national co-chairman and chief policy adviser of Trump's campaign, Sam Clovis, said last week that any Obamacare alternative "must be bipartisan." To anyone who has been engaged in the Obamacare debate, this is the clearest signal yet that Trump isn't serious about repeal. Bipartisanship won't lead to the repeal of Obamacare; it will lead to "fixing" Obamacare—which, in truth, can't be fixed—until such time as it finally gets overthrown in favor of a full-fledged government monopoly.

It is telling to contrast the statements of Team Trump with what a determined and genuine effort to repeal Obamacare would really look like. To revisit for a moment how Obamacare became law, after Massachusetts voters replaced the newly deceased Ted Kennedy with Republican Scott Brown (such was the degree of voters' opposition to Obamacare), Democrats no longer had the votes to pass Obamacare through the normal legislative process. So they relied on a process called "reconciliation," which allows budget-related measures to pass the Senate with a filibuster-proof simple majority vote. The Democratic-controlled House was therefore able to make changes to the Senate-passed version of Obamacare and get their changes passed back through the Senate. Without reconciliation, there would have been no Obamacare. As it was, Obamacare passed the House with only three votes to spare.

The way to repeal Obamacare is to use that same reconciliation process, while playing hardball like Obama and his congressional allies did. The Democrats passed Obamacare without a single Republican vote, and Republicans must expect to repeal it without a single Democratic vote. Republicans need to advance a winning alternative to Obamacare, without which repeal will be impossible, and then vote to repeal Obama's signature legislation and replace it with that alternative—using reconciliation.

Obamacare's individual mandate is plainly fair game for repeal under reconciliation—thanks especially to the Supreme Court's tortured ruling that the mandate is actually a tax—since taxes are clearly budget-related. As for what else is budget-related, it is helpful to review the arguments that the Obama administration made when the Court heard the main Obamacare case in 2012. The administration then argued that if the Court struck down Obamacare's individual mandate as unconstitutional, it must also strike down its preexisting-conditions mandate. Otherwise, the administration said, those who buy such "insurance" would disproportionately be older and sicker, and health costs would skyrocket. (That is actually what has happened under Obamacare even with these mandates operating in combination.)

Republicans must therefore make the case to the Senate parliamentarian that—according to no less of a source than the Obama administration—the preexisting-conditions mandate in the absence of the individual mandate would cause health costs to skyrocket and hence would have a budgetary effect. Moreover, the burden to include a cost-related provision in a reconciliation bill (as Republicans would be trying to do) should certainly be lower than the burden to strike down that same provision as unconstitutional (as the Obama administration was arguing should be done in similar circumstances).

If, despite the clear linkage between the individual mandate and the preexisting-conditions mandate, the Democratic-appointed parliamentarian, Elizabeth MacDonough, rules that the preexisting-conditions mandate isn't budget-related and therefore cannot be repealed via reconciliation, Senate Republicans need to be prepared to overrule or, if necessary, replace her. Needless to say, all of this will require a great deal of political capital and political courage. So the president's full commitment will be crucial.

Such full commitment is a far cry from being for Obamacare's preexisting-conditions mandate, for Obamacare's Medicaid expansion, and against an alternative doesn't have Democratic support. To insist on bipartisanship in this context is both naïve and dangerous. The Democrats already passed Obamacare into law. They did so in defiance of public opinion and without any Republican votes. Why would they now support its repeal? Aside from a few sensible defectors, there will be no joining of hands in this effort.

Obamacare is at the core of Obama's project of "fundamentally transforming the United States of America." Without being committed to Obamacare's repeal, one is guaranteeing Obama's wider victory. Yet despite his lament that "we never win anymore," Trump shows no interest even in trying to win this historic battle—and no recognition of what it will require.