Many liberal pundits were ecstatic about the spot, proclaiming that Democrats finally understood how to win on Obamacare. But Democratic strategists said that wasn't the most important element; the ad is "very much about Mark telling his personal story," and not about making a pro-Obamacare argument, said Justin Barasky, a spokesman for the Democratic Senatorial Campaign Committee.

"This is not a response ad by any means; this is a bio ad, this is an ad about who he is. They would have run this ad regardless of what the politics of ACA are," a Democratic strategist said.

Similarly, Senator Kay Hagan has made the law's Medicaid expansion a key component of her bid for reelection in conservative North Carolina, which has rejected the coverage expansion.

Democrats' emerging confidence comes as the law is taking a smaller role in Republicans' attack ads. GOP candidates and allies in a handful of states—including North Carolina—have shifted from an all-Obamacare-all-the-time advertising strategy to one that incorporates Obamacare into a larger message about jobs and the economy.

All those trend lines are pointing in the same direction, but that doesn't mean Democrats have suddenly won the upper hand on Obamacare.

First of all, it might not work. Pryor and Hagan have both backed into health care: He embraced it as a way to tell a personal story, and Hagan's focus on Medicaid is one that should resonate with the Democratic base in her state. But there's still a pretty good chance that Pryor and Hagan—like many of their colleagues who also voted for Obamacare—will lose.

And despite the excitement Pryor's ad stirred up on the left, Jennings said he doesn't expect to see a rush of Democrats taking the same tack. Candidates have other issues they'd rather focus on, he said, and there's "some health care fatigue out there"; after five years of bitter partisan fighting about Obamacare, polls show most people are ready to move on.

The same polls also show that the law remains unpopular and poorly understood.

In the most recent monthly tracking poll from the Kaiser Family Foundation, the law's favorability rating held steady at 39 percent, while its unfavorable rating rose to an all-time high of 53 percent. The same survey has found that Republicans' opposition to the law is much more passionate than Democrats' support for it. And even though the public at large wants Congress to fix the law rather than repeal it, a majority of Republicans said they'd prefer to keep the focus on repeal.

Those attitudes are so entrenched that Democrats may never hold an advantage on Obamacare. But polls also show that voters like many specific elements of the law—which is probably why Pryor didn't mention it by name when he described one of its central provisions.

In Kaiser's March poll, the most recent to survey individual components of the law, 70 percent of voters said they approve of a policy requiring insurers to cover people with preexisting conditions. Just 54 percent, though, knew that policy was part of Obamacare. Likewise, in the most recent survey, just 37 percent knew that the law offers consumers a choice among private insurance plans.