Republican candidates have now taken to the debate stage twice this election season to prove their conservative bona fides. They've repeatedly swore to unravel President Barack Obama's legacy. But there was one place they've barely gone: repealing Obamacare.

This is a party that spent five years casting vote after vote after vote to repeal Obama's signature domestic policy accomplishment. Just two years ago, Republicans shut down the government in an attempt to defund the law. The Tea Party itself was born out of calls to end Obamacare.

But last night, candidates mentioned Obamacare exactly six times during the first debate. Only two candidate, Scott Walker and Ted Cruz, have even uttered the Republican rallying cry: "Repeal Obamacare." The near-complete absence of Obama's health overhaul is remarkable.

The rhetorical shift shows a fundamental change in the calculus of Obamacare: It's one thing to talk about dismantling a theoretical law. It's another to take away insurance that tens of millions of Americans now receive. And that's exactly where Republicans are in 2016. So while Obamacare has barely made it onto the stage, it might just be the biggest winner of the debate season so far.

What a difference four years makes

In 2012, vows to repeal Obamacare were ubiquitous in the Republican field. As longtime health reporter Julie Rovner noted during the last campaign, "You can barely listen to Mitt Romney make a speech or give an interview without hearing some variation of this vow."

In August 2011, Republican presidential candidates took time to outline how, exactly, they'd wipe the law off the books. Romney promised to grant waivers to all 50 states (a legally questionable move but, nevertheless, a strategy!). Minnesota Rep. Michele Bachmann talked electoral strategies.

"As the nominee of the Republican Party, I also will not rest until I can also elect an additional 13 senators who agree with me so we'll have a filibuster-proof Senate and we can actually repeal Obamacare," Bachmann promised on stage.

There was none of that at last night's debate. About half the candidates committed to the idea of ending Obamacare. Donald Trump said, "We have to end Obamacare"; Jeb Bush mentioned getting rid of the law as a way to achieve his goal of 4 percent annual economic growth.

Nobody went past that. This year, no candidate has talked about waivers or elections or the actual obstacles to Obamacare repeal. Candidates didn't face questions about how far they would go to oppose the law or whether they would, for example, shut down the government again.

Obamacare repeal has, for five years, been a central character in Republican rhetoric. Last night it got relegated to a bit part.

Obamacare's secret weapon: its enrollees

During the 2012 Republican debates, Obamacare was an abstraction with few real beneficiaries. Nobody had actually signed up for Obamacare; they couldn't until 2014. Obamacare enrollees, back then, were about as common as unicorns.

Now Obamacare has an army: More than 10 million Americans get coverage through the health law's marketplaces and millions more through Obamacare's Medicaid expansion. America's uninsured rate has tumbled since the last time Republican presidential candidates took to a debate stage.

"In 2012, it was still all hypothetical," the Kaiser Family Foundation's Larry Levitt says. "There's a real difference between taking away subsidies some hypothetical person might get in the future versus real people who are actually getting that help."

Talking about Obamacare repeal is riskier and more complex than it was back in 2011. For one, Republicans would need to come up with a plan for dismantling Obamacare's infrastructure, how exactly they would take down the marketplaces, and what, if anything, they'd put in its place.

It's already pretty clear Republicans do not want to do this: When Obamacare's subsidies were at risk before the Supreme Court this spring, legislators tripped over themselves issuing plans to extend the health-care law's subsidies should the justices strike them down.

Those proposals implicitly acknowledged that it would be bad for Republicans to allow millions of Americans' tax subsidies to dry up, even though legislators still staunchly oppose the law. That would be true if the Supreme Court had struck down Obamacare's subsidies — and especially true if Republicans proposed doing so themselves.

Expect Obamacare to come up in the general election — because Democrats will talk about it

Democrats, meanwhile, will likely be more enthusiastic to discuss Obamacare than ever. In 2010, the party largely shied away from the health law after weathering bruising debates in Congress over "death panels" and abortion.

And even in 2012, Obama could mostly just talk about his health-care law in the hypothetical — not what it had done for Americans, but what it could do in the future.

Now that's different: The eventual Republican nominee will face question after question from Democrats about what he or she will take away from Americans — questions that simply couldn't be asked in 2012. At the Atlantic, David Frum has argued this could become the type of question that defines the 2016 election:

The next presidential election, like the last, will be decided by whether Democratic-leaning groups show up at the polls in large numbers—and maybe, at the margins, by whether the last few single percentage points of undecided voters choose "change" or "more of the same." For those economically stressed toss-up voters—for the younger voters who sometimes show up and sometimes vote—the tipping point issue won’t be foreign policy...It will be that single haunting question, "Will I lose my insurance?" If they don’t hear a clear and convincing "No," they’re going to assume the answer is "Yes"—and most likely, vote accordingly.

Obamacare is delivering health care to millions of people. That wasn't the world we lived in during the 2012 election, but it is the one we live in right now. And Republican candidates, whether they like it or not, are adjusting their political rhetoric to this new reality.

Watch: Why primary debates matter more than the general election debates