By Jeff Mason

WASHINGTON (Reuters) - If you can't beat 'em, join 'em.

President Barack Obama's campaign has embraced the term "Obamacare," seeking to turn the negative name Republicans assigned to his healthcare reform effort into a positive branding tool just as the Supreme Court studies the law's constitutionality.

"Happy birthday, Obamacare," Jim Messina, the president's campaign manager, wrote in an email to supporters last week to note the anniversary of the reform becoming law.

"If you're tired of the other side throwing around that word like it's an insult, then join me in sending a message that we're proud of it," he wrote.

David Axelrod, the president's top campaign strategist, was more blunt.

"Hell yeah, I like Obamacare," he said in an email to Obama supporters, encouraging them to express the same sentiment by clicking on a link to a campaign website and typing in their email address and zip code.

It was not always this way.

The White House has referred studiously to Obama's signature legislative accomplishment as the "Affordable Healthcare Act" for most of the two years since it passed.

Meanwhile, Republicans coined "Obamacare" to tie the president to the law, which polls show is still very unpopular with many Americans.

With the Supreme Court starting three days of arguments on Monday in a process that will determine the 2010 law's fate, Obama's advisers sought to shift from defense to offense in influencing public perception of the law.

"They're calling the bluff," said Allen Adamson, a brand expert at Landor Associates.

"For the people who like it, they're going to instantly associate it more directly with Obama. And for the people who don't like it, (it's) not going to make a lot of difference."

STRATEGY SHIFT

Republicans were skeptical the shift in strategy would work.

"If Obama truly wanted to own Obamacare they would have done it long ago and not just when it became painfully obvious he's losing the battle," said Kirsten Kukowski, spokeswoman for the Republican National Committee.

"Republicans are clearly going to use Obamacare against the president as a law that is unpopular, unconstitutional, will raise taxes, will cost more than we were promised and won't do anything about healthcare costs."

Republican presidential candidates regularly use the term on the campaign trail to tarnish the president.

Rick Santorum, the former U.S. senator from Pennsylvania, has tied it to a similar law championed by Mitt Romney, the front-runner for the Republican nomination who introduced healthcare reform in Massachusetts when he was governor there.

"Romneycare is a government-run healthcare program," Santorum said during a recent speech in San Antonio. "It's the template for Obamacare."

The Obama campaign acknowledges its shift on the word.

"Republicans have spent millions trying to make Obamacare a negative term," said campaign spokeswoman Kara Carscaden. "Our supporters have taken the word back and are ready to defend health reform against the Republicans' attempts to end it."

T-shirts that say "I like Obamacare" are available on the campaign's website for $35.

Buttons with the same message go for $5.

The White House, brushing away its fondness for proper names, has also taken on the Obamacare brand.

"One thing I'm confident of is, by the end of this decade, we're going to be very glad the Republicans termed this Obamacare, because when the reality of health care is in place, it's going to be nothing like the kind of fear-mongering that was done," said David Plouffe, a senior White House adviser, on ABC's "This Week" program on Sunday.

And Obama himself, not for the first time, seemed to embrace the term at a recent fundraiser, saying it showed that he cared.

(Editing by Doina Chiacu)