WASHINGTON (AP) — One of the most vulnerable Senate Democrats is standing by his vote for President Barack Obama's health care law, a fresh sign that the unpopular mandate may be losing some of its political punch.

In an ad released this week, two-term Arkansas Sen. Mark Pryor says he voted for a law that prevents insurers from canceling policies if someone gets sick, as he did 18 years ago when he was diagnosed with cancer. That prohibition on ending policies is one of the more popular elements of the 4-year-old law that Pryor never mentions by its official name — the Affordable Care Act.

"No one should be fighting an insurance company while you're fighting for your life," Pryor says. "That's why I helped pass a law that prevents insurance companies from canceling your policy if you get sick, or deny coverage for pre-existing conditions."

The law, dubbed "Obamacare," remains divisive, vilified by Republicans as big government run amok and a relentless hit on a sputtering economy. House Republicans have voted some 50 times to repeal, change or scrap the law, and the GOP is betting Americans' opposition to the law will be a great motivator in November's midterm elections.

The Obama administration insists the law is accomplishing its main goal — providing health care coverage to millions of Americans who lack it, with some 8 million enrolled. In Arkansas, for example, the share of uninsured residents dropped about 10 percentage points — from 22.5 percent in 2013, to 12.4 percent in the middle of this year, according to the Gallup-Healthways Well-Being Index.

Pryor is locked in a close race with first-term Republican Rep. Tom Cotton in a state that voted overwhelmingly for GOP presidential nominee Mitt Romney in 2012 and John McCain in 2008. Despite its Republican leanings, the state does have a Democratic governor.

Pryor's latest ad made political sense to Robert Blendon, a public opinion analyst at the Harvard School of Public Health.

"Democrats like this bill," Blendon said. "There's a big mistake that nobody likes this bill. They really like it lot and there are features of it that are incredibly popular with Democrats or more moderate independents."

Mobilizing core voters to go to the polls is crucial in low-turnout midterm elections. Pryor's embrace of the law sends a message to Democrats that a law they like could disappear if he loses his seat, Blendon said.

Republicans need to gain six seats to secure the majority for the remainder of Obama's term. Arkansas is one of the GOP's top targets.

Campaign ads have reflected the waning of health care as an issue. Commercials from candidates and the party organizations have focused on veterans, bipartisanship and attendance at committee hearings while Republican-leaning outside groups such as Americans for Prosperity, the U.S. Chamber of Commerce and American Crossroads still use many of their spots to hit Democrats for backing the health care law.

"The campaign committees are smart enough to realize that they're going to have to run on something besides a position that involves taking access to health care for millions of Americans," said Jim Manley, a former Senate Democratic aide who maintained that the election was always going to be about jobs and the economy.

Republicans in competitive races acknowledge that the law is a reality.

In southeast Pennsylvania, where biotechnology and pharmaceutical companies grab space along Route 202 in Chester County, GOP candidate Ryan Costello offers a pragmatic take on the health care law.

"In my opinion, this president is not going to repeal Obamacare in the next two years," says Costello after greeting families and volunteers at a community fair in Malvern, Pennsylvania, and speaking to business leaders.

Costello talks about "patient-centered, consumer-driven health care" and gives high marks to the Medicare Advantage program. If elected to the open seat currently held by six-term moderate Republican Rep. Jim Gerlach, Costello has an idea on how to fix the health care law.

"You're not going to say here's our 17 things to fix, it's all going to be in this bill," he said. "We're going to have to do it one by one. If you do that way, I think you're going to be more likely to get Democratic support."