STERLING, Va. — Speaking to one of the loudest crowds he's addressed in weeks, Mitt Romney made little effort to tamp down conservative enthusiasm for Thursday's expected Supreme Court ruling on the Affordable Care Act.

"My guess is they're not sleeping real well in the White House tonight!" he joked shortly after taking the stage, drawing loud cheers. "And that's the way it ought to be!"

Taking a cue from the energetic crowd, the candidate continued: "The decision is about whether Obamacare is Constitutional, whether it passes Constitutional muster... We already know it's bad policy and it's got to go!"

He wasn't the first one in the room to bring the subject up. The candidate was first introduced by Virginia Attorney General Ken Cuccinelli, a Tea Party hero, who began his remarks by excitedly reminding the crowd that the Court's ruling was expected in "less than 24 hours." Virginia Governor Bob McDonnell then called Obamacare "the greatest power grab in the history of America by the Federal Government."

All told, Romney spent the first eight minutes of his 18-minute speech railing against the health care policy — and the rally-goers ate up every word of it. But drowned out by the roaring applause and red meat rhetoric was the cold fact that, if Obamacare is overturned Thursday, the conservative vigor it inspired may vanish with it.

Opposition to Obama's health care policy has been the animating force of the Republican base since 2009, when the first waves of the Tea Party movement washed into Congressional town hall meetings across the country. It was the central issue of the 2010 midterms, when freshman Republicans flooded the House of Representatives. And throughout the GOP primaries, Romney's most reliable stump speech applause line has been his pledge to repeals Obamacare.

But if the Court strikes down the policy — effectively robbing Romney of the issue — it's unclear whether he'll be able to fire up his crowds with the mere mention of Obamacare.

Romney has already indicated that if the law is overturned, he will cast Obama as an ineffectual failure who squandered his first term on passing an unworkable piece of legislation. But that won't change the fact that for three years, "Obamacare" has functioned as an automatic press-for-applause button in conservative crowds. And on Thursday, it may be disabled.