A Mitt Romney-Rick Santorum ticket? Don’t rule it out.

It’s Southern Super Tuesday, and Rick Santorum and Mitt Romney have been dishing up the insults fast and furious ahead of crucial primaries in Mississippi and Alabama.

On Tuesday, Mr. Santorum went after Mr. Romney’s record as a venture capitalist – a line of attack that he had avoided before.

“Governor Romney has a career as an investment banker and someone who's a private equity guy on Wall Street. I'm not too sure that necessarily commends you well to be president of the United States,” Santorum said on the nationally syndicated show “Kilmeade & Friends,” according to MSNBC.

On Monday, Santorum suggested that Romney is a “socialist,” because “Romneycare” was “a government takeover of one-sixth of the economy.” Santorum was reacting to Romney’s assertion on Fox Business Network earlier in the day that Santorum isn’t conservative enough on fiscal matters.  

Romney had been asked whether he would choose a running mate who is more conservative than he, if he wins the nomination.

"Well, that would preclude Rick Santorum," Romney said. "I find it interesting that he continues to describe himself as the real conservative. Rick Santorum is not a person who is an economic conservative to my right. His record does not suggest he has the fiscal conservative chops that I have."

Santorum reacted with incredulity.

“I’m too liberal?” Santorum said Monday night in an interview with ABC News. “This is the imaginary world of Mitt Romney’s ideology. It’s just sad.”

Romney and his surrogates have repeatedly attacked Santorum for aggressively pursuing earmarks for his home state of Pennsylvania when he was a senator, and for voting for colleagues’ pet projects, such as Alaska’s infamous “bridge to nowhere.”

So, should we cross Santorum off the list of potential running mates for Romney, if the former governor of Massachusetts hangs on as front-runner and wins the Republican nomination?

Hardly. Heated rhetoric is part of the game, and when the time for choosing a running mate comes, the slate is essentially wiped clean. After all, Ronald Reagan chose George H.W. Bush as his running mate in 1980, even after Mr. Bush derided the Reagan economic proposals as “voodoo economics.” Mr. Reagan’s selection of Bush also represented a bit of ticket-balancing, as the conservative Reagan reached out to the more moderate Bush to reassure the electoral center.

Reagan also engaged in another time-honored tradition: choosing a running mate from the field of candidates who had challenged him for the nomination. The advantage there is that the running mate has already been scrutinized by the press and gone through the rigors of debates and high-profile interviews. When John McCain selected Sarah Palin in 2008, the then-Alaska governor had only been lightly vetted by his campaign and had no experience on the national stage, leading to some major stumbles.

True, a Romney-Santorum ticket pits two white guys against the first African-American president. But Santorum, at least, is Italian-American. Putting a minority or a woman on the ticket who is not widely seen as ready or qualified for the second highest office in the land might look like a rerun of 2008.

Santorum, a former senator from Pennsylvania, helps balance out Romney in other ways. Though both are northerners, Santorum has more populist appeal with blue-collar voters than the wealthy Romney. And while Santorum is Catholic, he also connects well evangelicals, many of whom are uncomfortable with Romney’s Mormon faith. Santorum also brings solid credentials as a social conservative to the table, in contrast with Romney’s evolution rightward on social issues that makes some Republicans uneasy.  

Putting Santorum on the ticket could also help put Pennsylvania, a battleground state, in the Republican column.