Consider a few numbers from last night’s voting and from these same contests four years ago.

Missouri: Four years ago, Mitt Romney got about 172,000 votes out of 589,000 cast. Last night, Romney got around 64,000 out of roughly 233,000 cast.

Minnesota: Four years ago, Romney drew 26,000 votes out of 63,000 cast. Last night—just 8,000 out of around 47,000 cast.

Colorado: In 2008 Romney won 42,000 votes out of 70,000 cast. Last night he got 23,000 votes out of 65,000 cast.

Across the board, then, Romney got fewer votes than he did last time, and each of these three contests drew fewer voters. Missouri drew a vote less than half the size of 2008—partly of course a function of the fact that yesterday’s vote ended up being a nonbinding beauty contest. But still, enthusiasm in general is down (this has been true of turnout in most states), and enthusiasm for Mitt Romney is down. That’s trouble.

Romney will still be the nominee, seemingly, but he has some big problems right now. Rick Santorum has some attractive qualities for GOP voters, but he has no real business being a top-tier contender. I know I wrote in December that he deserved his moment in the sun, but what I meant by that was, crikey, if Republican voters were willing to make Herman Cain their frontrunner for a while, then surely they ought to test-drive Santorum, who is conservative and at least knows some things about policy. But as a possible nominee? This is like Bill Richardson having swept three states against Barack Obama and Hillary Clinton in 2008. Imagine if that had happened. It would have said: boy, Democratic voters really don’t like their top choices.

At this point, it’s fair to start questioning how Romney can really unite this party behind him eventually and make people enthusiastic about him. It sure doesn’t look, on the evidence of last night, like he’ll seriously be able to compete with Obama in Colorado or Minnesota. Even Missouri, which Obama lost by 0.1 percent last time, looks problematic for him right now. But the Romney contests to watch in some ways aren’t against Obama or Santorum or Newt Gingrich, but against himself and against history—his numbers, and the overall turnout numbers, from last time. If he doesn’t show impressively in Arizona and Michigan on Feb. 28, GOP power brokers are really going to start wondering what they’ve bought into.