Dan Balz sets up the question for Mitt Romney's campaign:

Can he make this campaign all about the economy and President Obama’s stewardship in office? Or will he find himself in constant conflict over his Massachusetts health-care mandate, his conservative credentials, his “authenticity” and questions about whether he can connect with and truly rally the entire Republican Party?

Ooh, ooh, call on me! I have the answer: No! Has any candidate in a competitive primary ever managed to successfully sustain the focus entirely on his contrast with the opposite party's president, rather than on his contrast with his competitors for the nomination? Of course not.

Meanwhile, National Journal reveals that Romney's position on economic stimulus differed only marginally from the Obama administration's:

[A]t the dawn of Obama's presidency, as the financial crisis rocked the global economy and the United States shed jobs like mad, Romney pushed for a stimulus plan that mimicked Obama’s in philosophy. Namely, he advocated the idea that the federal government should prop up the free-falling economy by plunging itself further into debt. ...
Early in 2009, Romney said Congress was “rightly focused on a stimulus to stop the economic decline and end the recession,” although he hedged his endorsement with a caveat that returning money to taxpayers would give the stimulus a “bigger bang for the buck” than federal spending.
Asked specifically on CNN in January 2009 whether he supported Obama's call for a $750 billion stimulus, Romney did not object to the size or composition of the plan. Instead, he expressed general support for stimulus, citing a loss of $400 billion in consumer spending from the economy. "Government can help make that up in a very difficult time," he said, adding later: "I’d move quickly. These are unusual times."
At several points, Romney argued for increased spending on infrastructure, defense, and energy projects during his January 15, 2009, testimony to a Republican economic stimulus working group. The Obama stimulus was stocked with infrastructure and energy spending, in particular. Romney also called for allowing businesses to deduct capital purchases for two years, a measure Obama signed into law.
“We are presented with economic peril unlike anything we have faced during our lifetimes,” he said in written testimony. “I do indeed believe that careful, skillfully crafted stimulus can improve the prospects for recovery.”

Like Romney's health care plan, this was perfectly mainstream conservative thought in 2008. But in a party has embraced radical Paul Ryan-ism, yesterday's mainstream Republican is today's socialist.

Incidentally, this kind of makes me wish Romney won the 2008 election -- we'd have a stimulus and health care reform, and possibly cap and trade, and Republicans rather than Democrats would be bearing the political brunt of the aftermath of the financial crisis. 

In an article arguing persuasively that Romney's health care problem is unsolvable, National Review's Ramesh Ponnuru argues that Romneycare displays a lack of conservative fervor:

Romney has a reputation for being willing to say anything to advance his political career, but one comment he made in a mid-May health-care speech cannot be doubted: He believed that his health-care plan was in the best interests of Massachusetts in 2006, and he believes it now. For a right-leaning businessman with no conservative philosophical roots, no great familiarity with the range of conservative thinking on health care, and no deep skepticism about the way politics and bureaucracies work, the plan must have had strong appeal.
The mandate would force people to behave responsibly: No more would people be able to pass their costs to the state by going without insurance and getting care from emergency rooms required by federal law to treat them. A “connector” or “exchange” would ingeniously allow for the growth of an individual market within the constraints of a federal tax law that favors employer-based coverage. Employers would give their employees money, and they would then choose among insurance plans offered by private companies through the connector. The federal government, through Medicaid, would relieve Massachusetts taxpayers of some of the costs of increased coverage. Romney could tell himself that the plan fostered competition and kept the private sector involved if not in charge. And while most conservative health-care experts were lukewarm, at best, about the proposal, it enjoyed the enthusiastic support of the Heritage Foundation, which had long favored (and had to some extent originated) many of its elements.

Note that in the first paragraph, Ponnuru says Romney had "no great familiarity with the range of conservative thinking on health care," and in the next he points out that Romney's plan had the enthusiastic backing of the Heritage Foundation. That's conservative health care thinking! It's certainly true that some even more right-wing conservatives pushed health care ideas to the right of what Heritage, and most Republicans, advocated from the early 1990s through 2009. But very few people in public life took these ideas seriously at the time Romney crafted his health care plan.

Romney is evidence that Obama's policies -- on economic stimulus, health care, the auto bailout, and pretty much everything -- are anything but the extreme socialism they now portray them as. His nomination would undercut their claims daily, and demonstrate it is the GOP, not Obama, that is proposing a radical new direction for the country. That's why they can't nominate him. Now, Republicans don't process the thought that way. In their minds, Obama's policies are truly radical, and their party somehow failed to grasp this radicalism until Obama took office. But that is the dynamic at work.