Once major social policy changes take root, it’s almost impossible to unravel them. Why?

Having ducked the need to replace Obamacare, Republicans can now bitch and flail and face Washington’s most potent force: the status quo. The Affordable Care Act is now the de facto status quo as a result of the Supreme Court’s decision last week. And smart Republicans should know you usually get only one shot at the king.

They tried by concocting a theory that was less about substance than semantics: What did an exchange “established by the state” really mean?

Now they can rail against Obamacare and “activist” judges. GOP presidential candidates can make that a campaign refrain and thus let the party keep the issue without owning the outcome of the decision they sought, namely more than 6 million losing their health care.

But do they really have a chance to change, even repeal, the law?

“It’s hard to fully repeal it, but it is not that hard to change it substantially. The GOP has to have a strategy of fixing it, the more the better,” says John Feehery, a Republican consultant and former top aide to former House Speaker Dennis Hastert and House Minority Leader Tom DeLay.

Changing it substantially will be hard for a simple reason: the history of entitlements screams out that the longer a law is in place, the harder it is to ditch or dramatically alter it.

Indeed, there’s a lot of research about what the political scientists call “positive feedback loops” when it comes to policy.

The academics can make it sound complex but it isn’t: You pass a law, constituencies who rely on it are born and they and others proceed to resist changes.

“Social Security is the classic case, of course,” says Andrew Rudalevige, a political scientist at Bowdoin College.

There are a few examples to the contrary. For example, interest groups fought ferociously to regain their tax breaks and undermined a historic bipartisan tax reform in 1986 crafted under President George H.W. Bush.

But Social Security and other transformational changes, such as transportation deregulation (trucking and airlines), proceeded apace and suggest why Obamacare “has staying power,” as Rudalevige puts it.

It’s unleashed market forces that are hard to undo.

If you doubted that, be informed that HCA Holdings Inc., Tenet Healthcare Corp. and Community Health Systems Inc. shares all gained at least 8% shortly after Bloomberg News first broke word of the decision at 10:08:09 a.m. Eastern Standard Time on Thursday (beating Thomson Reuters by all of six seconds).

Further, Obamacare is pretty heavy on regulations, like Social Security and Medicare. Those become tough to alter, while creating a huge number of people with incentives to maintain their benefits.

It’s not guaranteed with metaphysical certitude but likely. One interesting qualifier is broached in the work of Andrea Campbell at Massachusetts Institute of Technology, who talks about welfare politics and its key constituents, the poor, voting infrequently and finding it harder to organize.

So one trick for Obamacare might be to make sure it insinuates itself into the middle class, which has more political clout and ability to resist change.

But it’s not as if even proponents don’t concede issues with the law, which did not alter the private health care system in any significant fashion.

“Basically the real challenge now is for delivery system reform — that is improving quality and lowering cost,” said Ezekiel Emanuel, an oncologist and medical ethics and health policy expert at the University of Pennsylvania and a former White House adviser on the health care law.

“The ACA galvanized that activity and is pushing it forward but a lot more needs doing. That will continue now without distraction of will ACA be repealed. That is good.

“And one would hope that there could be more bipartisan overlap to push initiatives that would accelerate the transformation. It is happening but could go faster.”

For sure. But, with a presidential campaign upon us, the near certainty is that even as more Americans may benefit from the law, Republicans will beat the drum.

And they’ll go after Hillary Clinton for her support, while she beats her own drum among minorities, especially Latinos, and gains advantage.

The bottom line will be the same: The GOP can hector all it wants. But, as with gay marriage, they’ve now lost the argument.

There’s a reason social policy rarely gets unraveled