Newt Gingrich and Mitt Romney: How different are they on the economy?

If Newt Gingrich has become the leading challenger to Mitt Romney, then a basic question is pertinent to their looming duel for the Republican presidential nomination: Where do they diverge on policy?

For all the talk about Mr. Gingrich as the new "anti-Romney," gaining traction with voters who see the former Massachusetts governor as failing the true-conservative test, the two have some major similarities when it comes to the economic issues that are central to the 2012 election campaign.

Both have staked out staunch conservative positions on some fronts: keeping the Bush tax cuts in place, repealing Obama's health-care reforms, eliminating the estate tax.

Yet both face some doubts from core conservative voters in their party. Mr. Romney takes heat for supporting a Massachusetts law mandating that individuals buy health insurance, but Gingrich has voiced support for that basic idea. Gingrich has caught flak for supporting a path toward legal status for many illegal immigrants, yet Romney also opposes blanket deportation.

The point of these comparisons isn't that the two offer carbon-copy platforms. Big differences do exist (coming in a moment).

But to the degree that primary voters focus on similarities, that could work in Romney's favor as he seeks to fend off Gingrich's rise. He can use the parallels to argue for his credentials as someone in the conservative mainstream of his party. And his campaign can argue that Romney's perceived flaws are shared by his rival.

Romney can count on other candidates, hoping for their own spot in the electoral sun, to put up their own opposition to the newly ascendent Newt. Some already have, with Rep. Ron Paul of Texas making news Thursday with an ad calling out the former House speaker from Georgia for "serial hypocrisy."

Gingrich's prospects, by contrast, may grow brighter the more he can be a salesman for policies to the right of Romney.

Here are some of the main differences on economic policy:

First and foremost, Gingrich comes off as the bigger tax-cutter, notably for businesses and wealthy Americans.

He proposes reducing the income-tax rate to zero on capital-gains and dividend income. Romney would also exempt such investment income from taxes, but only for households with incomes of $200,000 or less.

Gingrich also wants to offer taxpayers a radical choice: Pay taxes in the current system or opt for a flat 15 percent tax on income. That option would be particularly appealing to high-income households.

Still, Gingrich isn't proposing to scrap the current tax code entirely, as Gov. Rick Perry of Texas would do (with a 20 percent flat tax) or as business leader Herman Cain would do (with his so-called "9-9-9 Plan" and his longer-term goal of reliance on a national sales tax).

Gingrich would also reduce the tax rate on business income from 35 percent to 12.5 percent. Romney would set the corporate tax rate at 25 percent.

Another difference is on Social Security: Gingrich has proposed fixing the program's coming shortfall by empowering Americans with the option to invest in personal savings accounts. Romney has said possible reforms could include raising the retirement age or changing the way benefits are inflation-adjusted for high-income retirees.

The Gingrich campaign argues that Americans will opt into the personal-account system, because of its promise of higher lifetime benefits, in the process "drastically lowering [Social Security's] burden on taxpayers."

In all, Gingrich appears to occupy ground well to the right of Romney on several fronts.

As New York Times political blogger Nate Silver put it recently, Gingrich's score on a conservatism meter "would be fairly close to that of Gov. Rick Perry, which in my view represents something of a sweet spot for the Republican primary electorate: solidly conservative but not in Michele Bachmann territory."

What about positions on health care and immigration?

Gingrich says that, in supporting an individual health insurance mandate in the early 1990s, he was teaming up with the conservative Heritage Foundation against Clinton administration health care proposals.

But as recently as four years ago, though, he wrote approvingly of requiring "anyone who earns more than $50,000 a year to purchase health insurance or post a bond." That was in a larger commentary, which also espoused "market-based reforms," written in his role as founder of the Center for Health Transformation.

And this year he ruffled conservative feathers by joining liberals in criticizing a radical overhaul plan for Medicare proposed by Rep. Paul Ryan (R) of Wisconsin.

Instead of phasing out Medicare in its current form, Gingrich would allow seniors to choose a private-sector insurance system. "This would create price competition to lower costs," as well as giving individuals "greater options for better care," his campaign website says.

On immigration, Gingrich drew criticism from Romney and others for arguing for a kind of "amnesty" (the critics' word) for illegal immigrants. Gingrich's proposal, however, focuses on legal residency and stops short of flinging the doors of citizenship wide open.

"I do not believe that the people of the United States are going to take people who have been here a quarter century ... and expel them," Gingrich said. Fox News anchor Bret Baier recently asked Romney to clarify how his view differs from that of Gingrich. Romney didn't answer Mr. Baier directly.

At least one conservative pundit, former Education Secretary William Bennett, has rushed to defend Gingrich's position.

"Newt Gingrich's immigration plan could be a breakthrough moment for conservatives," he said Tuesday in a column for the CNN website. "It could be a new kind of signal from conservatives that we are not bound in an absolutist straitjacket when it comes to immigration reform."