WASHINGTON -- Former Gov. Mitt Romney laid out in painstaking detail Thursday his argument that the health care plan he passed in Massachusetts is fundamentally different from the national law that President Obama passed and that conservatives loathe. 

In a major speech in Michigan, a state that's an important part of his GOP presidential race, Romney, armed with a Power Point presentation, attempted to address conservative concerns about his health care program that supposedly served as inspiration to Obama's federal law.

"Our plan was a state solution to a state problem and his is a power grab on the federal government to put in place a one-size-fits-all plan across the nation," he said. 

"I know some of my more liberal friends don't find that a compelling difference," he said. "But those of us who believe the decision to make America a federalist system was not just a throwaway decision on an important fundamental element of what makes America such a successful nation and that is something of course is lost in ObamaCare."

While Romney has made this argument before in the last couple of years, this attempt was his more forceful yet as he laid out his case in a USA Today opinion article ahead of his speech. The intensity of his argument suggests that Romney's camp is fully aware that this issue could make or break his second bid for the White House. 

"If I am elected president, I will issue on my first day in office an executive order paving the way for waivers from ObamaCare for all 50 states," he wrote in the USA Today article. "Subsequently, I will call on Congress to fully repeal ObamaCare."

 Romney is trying to take a mulligan on the health care plan he passed in Massachusetts while offering a new federal version that conservatives can swallow.

But his GOP rivals aren't buying it.

"I greatly respect Gov. Romney and admire many of his personal and professional accomplishments, but his work to institute the precursor to national socialized medicine is not one of them," former Pennsylvania Sen. Rick Santorum said in a statement.

"Both Romneycare and Obamacare infringe upon individual freedom and exponentially increase the government's healthcare cost burden," he added. "Romneycare has, in fact, not made healthcare better or saved costs in Massachusetts. It's done just the opposite."

A report released this week by the Massachusetts Medical Society says under "RomneyCare," more than half of all primary care physicians in the state are no longer accepting new patients, and the average waiting time to see specialists is lengthening.

In 2008, Romney waited until weeks before GOP primary voting began to address a big vulnerability in his quest for the presidency -- his Mormon faith. Advisers now say that was too late to answer voter skepticism.

This time, Romney launched a full-court press to answer doubts about his health care plan and his Mormon faith. 

But instead of apologizing for his role in the Massachusetts law, which was enacted five years ago, Romney offered a defense of it. He argued that Republicans should repeal the federal law before most of its components take effect and replace the law with an alternative that gives states more of a say in the public's health care options.

Like the federal law, the Massachusetts plan requires individuals to buy health insurance and imposes tax penalties on those who don't. Both plans penalize small businesses above a certain size that don't provide coverage to their employees. Both rely on new taxes for some of the financing.

Since Congress approved the national health overhaul a year ago, Romney has struggled to answer criticism of his role in the Massachusetts plan and, despite the obvious similarities, has sought to explain how it differs from Obama's. He also has talked up its benefits; Massachusetts has succeeded in raising the number of insured residents to 97 percent.

It won't be easy for Romney to shake the ghost of health care.

The public's angst over the federal law has shown no sign of fading. An Associated Press-GfK poll in March found 82 percent of Republicans oppose the plan.

Rep. Debbie Wasserman Schultz, chairwoman of the Democratic National Committee, tore into Romney for seeking to repeal a law modeled on his state plan.

"I think what is unfortunate about Mitt Romney is that he doesn't even know who he is," she told ABC News. "The last thing that voters want is...someone who has no convictions. Mitt Romney has spent a number of years twisting himself into a pretzel trying to figure out which voters he's in front of and decide what positions he's going to take."

"He was the author of legislation that was very similar, if not close to identical, to the Affordable Care Act in Massachusetts," she continued. "And now he says in a speech today that he's going to support repeal and replace. What he's trying to do is repeal and erase his own record. And that's just simply not possible. It's not right."

The Wall Street Journal also cast doubt Romney's ability to gain the upper hand in the health care debate.

"The debate over ObamaCare and the larger entitlement state may be the central question of the 2012 election," the newspaper said in an editorial published Thursday entitled "Obama's Running Mate". "On that question, Mr. Romney is compromised and not credible. If he does not change his message, he might as well try to knock off Joe Biden and get on the Obama ticket."

But some Republicans say Romney is making the right move by addressing health care now.

"I think he's very smart to take the issue head on early in the primary process," said former Republican National Committee Chairman Ed Gillespie, who once served as counselor to President George W. Bush. 

The Associated Press contributed to this report.