For Rick Santorum, they are everything. If Santorum can pull off a victory in at least one of the three states — Minnesota seems to be his best shot — he has a rationale for going on. If he were to win both Minnesota and Missouri, where Newt Gingrich isn’t even on the ballot, Santorum would earn legitimate bragging rights and could make a plausible case that he is the more viable alternative to Mitt Romney in the coming showdowns.

Romney needs to stop this from happening. A Romney sweep tonight might not decide the nomination as such, but it would allow him to begin a consolidation process among Republican leaders who have stayed neutral up to now. (And since it would effectively knock Santorum out of the race, it would set up a Romney-Gingrich choice that Romney can win.) Romney carried Colorado four years ago. He needs to win it again, and by a bigger margin.

Tonight is also important to Ron Paul. He will keep going no matter what. But his strong early showings expanded his coverage in the media and amplified his message. He has been weaker in the more recent voting. Paul needs to show some strength in what are expected to be relatively low-turnout affairs to ratify his claim that he speaks for a significant minority in the GOP.

Speaking of turnout, Republicans really have to be troubled by the very low rate of participation in Nevada’s caucuses on Saturday. As Molly Ball reported on the Atlantic’s Web site, in 2008, “Romney took 51 percent of 44,000 votes in the Nevada caucuses,” while in 2012, Romney won “50 percent of the less than 33,000 votes, a decline in both vote share and turnout from 2008.” A win is a win, of course, but that’s a very large drop in turnout.

Not so long ago — and especially on Election Day 2010 — most of the energy in American politics seemed to be on the right end of the spectrum. Partly because many Republicans are unenthusiastic about their presidential field and also because the Tea Party seems demobilized (certainly compared with the recent past), the opposition to President Obama is far less vigorous. So watch turnout carefully tonight (or, more especially, on Wednesday, when all the precincts are counted).

With the president rising in the polls, Republicans more than ever need the kind of mobilization they managed in 2010 if they’re to have a chance in the fall. It’s an old political rule that may apply especially to Republicans in 2012: no energy, no victory.