Generally speaking, political campaigns don’t waste time or effort attacking competitors who pose no threat to their standing.  The Mitt Romney campaign has mostly focused on Newt Gingrich for its attacks, which made sense in December and January, as Gingrich had the poll standing and the cash to pose a serious threat — a threat fulfilled in South Carolina.  Now, however, the Romney campaign has a new target:

The campaign has sent out three press releases attacking the former Pennsylvania senator in the past 24 hours — and is trotting out lead-surrogate former Minnesota Gov. Tim Pawlenty to attack Santorum in a conference call this afternoon.

“Rick Santorum is a nice guy, but he is simply not ready to be President,” Pawlenty said in a statement released by the Romney campaign. Pawlenty also attacked Santorum for his record as “pork-barrel spender” who is not as conservative as he presents himself to be.

The new focus is a response to Santorum’s strong position leading up to Tuesday’s Minnesota caucus, leading Romney 29% to 27% in a Saturday poll by the Democrat-leaning Public Policy Polling.

Additionally, Santorum has been the only candidate who seems capable of tripping up the usually-unflappable Romney. In the two Florida debates, Santorum drew blood with attacks on the Massachusetts health care law Romney championed. The Romney campaign issued a separate list of Santorum’s “false attacks” on Romney’s signature legislative achievement in the Bay State.

As it happens, pork-barrel spending is one of the issues on which I disagree with Santorum, DADT being another.  Santorum has defended the practice in general in the same way that Ron Paul defends it, which is to highlight spending authorizations as a Congressional responsibility, and one that is in theory more accountable than allowing executive-branch agencies to make those decisions.  In theory, Santorum and Paul are correct.  In practice, however, Congress has had very little transparency on earmarks until very recently, and they ended up mostly being used to flex their muscles in home districts in order to boost their advantage as incumbents.  That has allowed a culture of corruption to flood Washington from both Republicans and Democrats alike, and Congressional reforms to the process have been halting and half-hearted at best.

That specific attack line indicates that Romney has begun to see Santorum as an increasing threat.  Nate Silver agrees, to a point:

A contiguous block of eight swing states containing 95 electoral votes — Indiana, Iowa, Michigan, Missouri, Minnesota, Ohio, Pennsylvania and Wisconsin — determine the winners and losers in most presidential elections. When at least six or seven of these states are added to the state bases of the Democratic or Republican candidate, he or she is all but guaranteed a victory. (Barack Obama won seven of them in 2008). Only when they are about evenly divided, as in 2000 or 2004, do swing states in other parts of the country — like Nevada or New Hampshire or Florida — tend to make much difference.

Mr. Romney lost Iowa to Rick Santorum, albeit by about the narrowest possible margin. He will have two more opportunities to win a Midwestern state on Tuesday, when Minnesota has its caucuses and Missouri holds a primary. (The Missouri primary does not matter for delegate selection: the state will hold a separate caucus for that purpose in March.)

Mr. Romney could be vulnerable in both states. A survey released on Sunday by Public Policy Polling, which has had fairly accurate results so far in the primary season, had Minnesota as a toss-up between Mr. Romney and Mr. Santorum, with Newt Gingrich and Ron Paul not far behind.

And in Missouri, where Mr. Gingrich is not on the ballot for the “beauty contest” primary, it had Mr. Santorum ahead of Mr. Romney, 45 percent to 34 percent.

Imagine that Mr. Romney were to lose both states. That would make him zero for three in the nation’s most important swing region. It would raise questions about his performance in Ohio, probably the most important state to vote on “Super Tuesday,” March 6. Polling there also shows a competitive race.

If Romney loses both, he would still have not lost any delegates.  None of the contests before Arizona this month produce delegate allocations except Nevada; they’re all non-binding.  But it would raise questions about Romney’s ability to connect in “flyover country,” and would highlight Santorum’s ability to play in the Midwest.

Bill Kristol believes a Santorum-Romney fight would produce a more “serious and constructive” battle in the GOP primaries:

In Minnesota and Colorado, the caucus system will result in a proportional allocation of delegates among the various candidates. But with polling showing Santorum even with Romney in Minnesota and second to Romney in Colorado, a strong showing for Santorum would do the most to slow the Romney juggernaut. It would also of course help Santorum’s chances to replace Gingrich down the road as the alternative to Romney—an outcome that, I suspect, might well result in a better race for the nomination and a healthier situation for the ultimate Republican nominee.

A final point: vote. The Romney-Gingrich slugfest of negativity seems to have produced a low turnout in Florida and Nevada. But the choice before you remains no less important than it was before all the negative ads started airing. Indeed, you who will vote tomorrow have a chance to get us beyond the unseemly spectacle of the last couple of weeks. You can put Romney on a likely path to the nomination. Or you can create the possibility of a serious and constructive Romney vs. Santorum race.

Obviously, I agree with this assessment.  Given the new focus on Santorum, it looks as though Team Romney now sees Santorum as a considerable threat.  We’ll see how Santorum responds, but he’s been undaunted by all of the twists and turns so far.