Mitt Romney was attacking Rick Santorum with everything he's got yesterday:

The Romney campaign released a barrage of opposition research on Santorum on Monday morning, the type of offensive tactic that had previously been reserved for Newt Gingrich and, before him, Texas Gov. Rick Perry. The former Massachusetts governor's campaign worked to link Santorum to pork barrel spending during his time in Congress, and touting his endorsement of Governor Romney in the last presidential race.

Why now? Because of the three races tomorrow—Colorado, Minnesota, and Missouri—Romney only has a decisive lead in Colorado. And he demonstrates a serious weakness in the midwest. Out of all the races so far, Romney has dominated with wealthy voters and only managed to tie, at best, once the voters' annual income dibs below two hundred thousand dollars a year. Santorum does better with poor voters, and so the Romney campaign is preparing to do to Santorum what they did to Gingrich in Florida—open up the sewage pipes and hope to make the whole field so toxic that poor voters decide not to bother.

I have my doubts about whether this attack will work on Santorum—he doesn't have Gingrich's mile-high victim complex, so he won't inflate the attacks into a cosmic injustice—but this kind of all-out negative attack suggests that Romney is still a fundamentally weak candidate. Even though the media has fallen for this bullshit "momentum" story out of Nevada, the nomination is still in doubt, and Romney's people know it.