There’s a simple explanation. Tuesday also marked the first day of open enrollment in new insurance exchanges under the ACA, and the health plans were signing up loads of new customers. After months of markets pricing in Obamacare’s tax and regulatory headaches, here was the windfall. Of course, the one-day performance of a handful of stocks can’t be dispositive for a law years in the making, and one that will take many more years before it reaches full steam. But it is a striking turnabout for an industry that four years ago funneled $86 million through the U.S. Chamber of Commerce to fund a campaign aimed at sinking the reform drive. That sum might be a rounding error on Wall Street; in Washington, it amounted to more than the industry’s main trade group had spent the entire year previous.

So what happened in between the fierce fight over the bill and the Tuesday debut of the law’s key feature? “Once the industry got a comfort level around how reform was going to be implemented, their view of it got less unfavorable,” says Sarah James, an analyst with Weedbush Securities. Put another way, after the law survived a Supreme Court challenge and a make-or-break presidential election, health insurers decided it was here to stay. Their best bet became adjusting their business models to deal with it. That hasn’t meant the industry is suddenly a champion of the law, or has given up trying to manipulate it. Indeed, the health sector has already spent more than $243 million on lobbying this year, according to the Center for Responsive Politics, much of it directed toward shaping the law’s implementation. Pressure from business groups led the Obama administration to push back by one year an important piece of the law — the mandate on larger employers to provide insurance. And corporate stakeholders will continue spending heavily on ads related to the law. Ads from insurers, for example, are expected to eclipse those from ideological groups and political candidates as the 2014 midterm elections heats up. But even those spots that play off anxieties about the law will be aimed primarily at enlisting new customers.

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It’s a lesson the Tea Party Republicans setting their party’s strategy have had a hard time learning. They should try. For one, the very fact of the industry’s shift makes their already quixotic attempt at dismantling the law even more remote. Consider the history: President Clinton’s 1993 health reform drive failed in large part because of united industry opposition. Obamacare squeaked through in part because the Obama White House was wily enough to learn that lesson and engineer a divide-and-conquer approach to industry that peeled off potential antagonists, namely the big drugmakers. Now that Republicans’ best-funded industry allies have abandoned them, they’re occupying an increasingly lonely position in the debate.

But beyond the practical considerations, there’s an object lesson here for the GOP in the attitude of their erstwhile fellow travelers in the health insurance business. Sheryl Skolnick, an analyst with CRT Capital Group, said health insurance executives fitfully came to a similar conclusion about the law: “This is not the way we would do it, but it’s the law of the land, and therefore we must accept it and work with what it is.” Better to do that and try to profit than waste energy and resources on a fight that’s already lost.