Former Pennsylvania Senator Rick Santorum's sweep of Tuesday's contests in Minnesota, Colorado and Missouri is the latest development to throw into turmoil the notion that Mitt Romney is the inevitable nominee to go up against President Barack Obama in November.

Rick Santorum (Photo courtesy of Gage media; Creative Commons)

Santorum was once considered a minor candidate in the race, with national poll numbers remaining in the single digits while other candidates' popularity came and went in waves. Now Santorum is enjoying a surge.

He's perhaps best known for his hawishness over foreign policy, especially relating to Iran, and his unflinching positions on social issues such as abortion and gay rights. He is, as the Atlantic Wire puts it a "culture warrior."

Santorum was voted out of his Senate seat in 2006 when Democrats took control of the House and Senate at the time that George W. Bush was in the depths of unpopularity.

What makes it most surprising is that, compared with Romney especially, Santorum has no money. Ezra Klein, economics blogger for the Washington Post, summarizes:

One lesson to draw from that: Money ain't everything. Because Mitt Romney has money. Mitt Romney has lots of money. Between the resources of the Romney campaign and the Romney-allied SuperPACs, Santorum isn't even competitive. And since Romney also gets more free media, Santorum, by this point, should pretty much be out of the game. But he's not. And that's because money -- and even media -- ain't everything.

How big is the money gap overall? Romney, according to Open Secrets, has raised almost $56 million so far. Santorum has raised just over $2 million. The contrast is also sharp when you look at the Super PACs. The largest Romney Super PAC has raised over $30 million. Santorum's biggest, the Red, White and Blue Fund, has spent more than $2 million but only raised about $700,000. Klein's point is that money isn't everything, but it's possible that Santorum wouldn't have a chance at all at the nomination if it were for Super PACs producing ads like this one.

The Super PAC has been largely bankrolled by billionaire mutual fund manager Foster Friess, and Santorum's friendship with him has raised eyebrows.

These events mean Romney will have to step up his game. Tuesday night may have been a wake-up call, writes Paul Begala at the Daily Beast:

Romney has more money, more national experience, more consultants, more staff. Heck, he even has better hair. His super PAC outspent Santorum's by a 40-to-1 margin. Forty to one. And yet Mitt Romney lost. He lost to a guy who lost his home state by 18 points the last time he was on the ballot there. There's a technical term in political consulting for a performance like that: it's called sucking. If Romney can't beat Rick Santorum, he needs to find another party to run in.

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