At a campaign stop in Texas, Santorum renewed his charge that Romney had compromised his ability to challenge Obama on the federal healthcare law by backing a similar plan when he was governor of Massachusetts that included a mandate requiring people to buy insurance.

"Romneycare is a government-run healthcare program, it's mandates, it's fines, it's insurance exchanges set up by the government," Santorum said during a speech at the San Antonio campus of the military-focused financial services and insurance company USAA.

"It's the template for Obamacare," Santorum said, using a term favored by Republicans to describe the federal healthcare law, Obama's signature policy achievement.

The attack is part of a new campaign offensive by Santorum to link Romney's Massachusetts plan, which he signed in 2006, and Obama's federal overhaul ahead of the law's two-year anniversary on Friday and next week's Supreme Court arguments.

"Romneycare is the fundamental difference between Mitt Romney's campaign and mine," Santorum said in a statement released before his San Antonio speech. "I have the credibility and the record to stand up to Barack Obama and Mitt Romney on the healthcare issue and say 'no'."

Romney has argued that his plan was a solution to a Massachusetts problem, not an effort by the federal government to apply a one-size-fits-all program across the country.

Santorum is waging an uphill campaign battle against Romney, who has more than twice as many delegates to the nominating convention as Santorum in the Republican race to pick a challenger to Obama in the November 6 general election.

Santorum, a staunch social conservative, has tried to rally conservatives who still harbor doubts about Romney because of the healthcare plan and other more moderate stances he took while governor of liberal Massachusetts.

"A DIFFERENT VISION"

In San Antonio, Santorum seemed to imply that voters might as well stick with Obama rather than switch to Romney.

"You win by giving people the choice. You win by giving people the opportunity to see a different vision for our country, not someone who is just going to be a little different than the person in there," Santorum said.

"If we are going to be a little different, we may as well stay with what we have instead of taking a risk in what may be the Etch-a-Sketch candidate for the future," he said, a reference to a Romney aide's comment that Romney could switch his conservative positions in the general election "like an Etch-a-Sketch."

Romney quickly responded with a statement blasting Santorum for saying voters might as well stick with Obama.

"I was disappointed to hear that Rick Santorum would rather have Barack Obama as president than a Republican. This election is more important than any one person," he said. "It is about the future of America. Any of the Republicans running would be better than President Obama and his record of failure."

During his speech in Texas, which has a primary on May 29, Santorum said the "Etch-a-Sketch" comment by Romney's senior adviser Eric Fehrnstrom showed Romney's true colors.

"No one can accuse me of that, and people say I can't win because of it," Santorum said. "I say I will win because of it. I'm someone who is real."

He also shrugged off the endorsement of Romney by former Florida governor Jeb Bush, saying the campaign is not about endorsements and money.

"I don't spend my time chasing around elected officials who are part of the system," he said. "I want to change the system."

The next nominating contest is on Saturday in conservative Louisiana, where two polls this week showed Santorum with comfortable double-digit leads over Romney.

(Writing by John Whitesides; Editing by Xavier Briand)

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