WASHINGTON Republicans in Colorado, Minnesota and Missouri will vote for presidential candidates today, and while Mitt Romney is favored to win, many conservatives appear eager to signal their unease with him by voting for Rick Santorum.

Social conservatives are particularly active in Minnesota, which will hold caucuses. One poll has Romney in a virtual tie with Santorum.

Polls are often poor predictors of caucus results, however, where attendance is a function of campaign organization and voter passion.

"Caucus attendees here tend to gravitate to the most conservative candidate," said former Minnesota Gov. Tim Pawlenty, a Romney supporter. Santorum has been emphasizing family and faith, painting himself as the race's true staunch conservative.

Santorum questioned Romney's conservative credentials Monday, while the Romney camp painted Santorum as aggressively spending federal money during his Senate years.

Public Policy Polling, a Democratic survey firm, found Santorum ahead of Romney by 29 to 27 percent in its Saturday poll, and called the race a toss-up. Not far behind were Newt Gingrich, a former speaker of the House of Representatives, with 22 percent and Texas U.S. Rep. Ron Paul, at 19 percent. Paul has campaigned hard in the state; Gingrich has made little effort.

Colorado's caucuses are less of a risk for Romney, though tea party candidates won statewide primaries there last year before losing in the general election. Romney has a strong state organization and has campaigned hard there in recent days, while Gingrich and Santorum aren't as well organized there.

Missouri also will vote today, albeit in a nonbinding primary that will select no delegates. They'll be selected later.