Mitt Romney returned to the campaign trail here Monday and trained his focus on President Obama, but his top aides and key surrogates fought to beat back a possible surge by Rick Santorum on the eve of Republican presidential contests in a trio of states.

Romney’s aides and a top surrogate, former Minnesota governor Tim Pawlenty, unloaded on Santorum over his support of taxpayer-funded earmarks during his years in Washington, calling him a “champion of earmarks.” The Romney team lobbed similar broadsides against Newt Gingrich ahead of last month’s Florida primary, but have let up somewhat as the former House speaker’s campaign has struggled.

Santorum threw some salvos of his own, assailing Romney’s health-care record as Massachusetts governor and saying he is “simply dead wrong on the most important issue of the day and should not be the nominee of our party.”

In a speech he delivered across the street from the Mayo Clinic in Rochester, Minn., Santorum added that a Romney nomination would be a “devastating thing” for Republicans, according to the Associated Press.

A couple of hours later, Romney campaign aides shared with reporters comments Santorum made in 2008 when he was backing Romney in that presidential race. “If you’re a conservative, there really is only one place to go right now . . . and that’s Mitt Romney,” said Santorum, a former senator from Pennsylvania.

The Romney team has not unleashed such attacks on Santorum since his narrow win in the Iowa caucuses last month. As of Monday, the campaign was not airing any paid negative advertisements about Santorum.

Romney did not mention Santorum or any other Republican rival in his Monday afternoon rally in Grand Junction. He preserved his rhetorical fire for Obama, telling a crowd of a few hundred here: “Based upon the president’s own standard, he has failed. He does not deserve a second term.”

But in an interview with WCCO Radio in Minneapolis, he said, according to the AP, that Santorum’s “policies are, in my view, those of many Republicans in Congress who went along with Democrats to raise the debt ceiling, to allowing earmarks and to growing the size of federal government to a level that is frankly choking off the capacity of our economy to grow at the rate it should.”

Santorum accused Romney of throwing “the kitchen sink” at him. “He simply goes out and tries to destroy,” Santorum told reporters in Minnesota. “I don’t think it’s going to work this time.”

Santorum finished last among the four major candidates in Nevada’s GOP caucuses on Saturday, but he has devoted considerable time to the three states that will vote Tuesday. He said Sunday that he thinks he can do “exceptionally well” in Missouri’s nonbinding primary and “very well” in caucuses in Colorado and Minnesota.

He hopes a strong finish can provide a fresh jolt to his candidacy, which has struggled to raise money and build a national organization that can compete with Romney’s more formidable operation.

As the campaign moved onto multiple fronts this week, the candidates were forced to choose where to focus their time and money. Gingrich spent Monday campaigning in Colorado and Minnesota, while Rep. Ron Paul (Tex.) held rallies in Minnesota.

Romney was scheduled to stump in Minnesota, but abruptly canceled those plans so he could spend time in Colorado, where he has a robust organization and is considered the favorite.

Although the three contests could give winners momentum, they are nonbinding and will not award delegates, according to the Republican National Committee. Colorado’s 36 delegates will be chosen at district conventions in the spring. Minnesota will award 40 delegates at district conventions in April. And Missouri’s primary is merely a “beauty contest” — the state’s 52 delegates will be chosen beginning with precinct caucuses on March 17.

Romney’s prospects for a clean sweep of the three states were unclear Monday. Gingrich did not make it on the ballot in Missouri, which could help Santorum consolidate conservatives. And in Minnesota, whose caucuses Romney won in 2008, public polling showed Santorum running strong, but polls have been inconsistent and are considered unreliable.

Both states will be important barometers for Romney’s support in Midwestern swing states, but the candidate’s advisers tried to play down his chances.

“It’s a relatively small turnout, and the caucus attendees here tend to gravitate to the most conservative candidate,” Pawlenty said in a conference call with reporters. He quickly corrected himself to say “the most perceived conservative candidate.”

Pawlenty noted that in 2008, about 60,000 voters turned out for Minnesota’s GOP caucuses. If a similar share participates this time, it would represent about 2 percent of the state’s 3 million registered voters. He said all four candidates could be “bunched together toward the top of the pack.”

Staff writer Felicia Sonmez in Washington contributed to this report.