Michigan, where the race has heated up with big ad spending from Romney and Santorum, was dark at the beginning of this week, but the campaign settled a formal presence there some 72 hours ago, the Santorum campaign source said.

Santorum holds an 8.2-percentage-point lead against Romney in Michigan -- the former Massachusetts governor grew up there -- and campaign sources say a win in the state would show their viability to Republican voters as the race heads to Super Tuesday.

Also, Ohio's crucial status in the GOP primary and the general election have drawn the Santorum camp to set up a makeshift headquarters in the state.

A key vehicle for Santorum's grassroots approach in the upcoming primaries, the campaign source said, are locals. Santorum has depended heavily on resident volunteers familiar with the political landscape of their precincts, districts and regions where they live.

"We're ramping up our presence, but we've got armies on the ground," the source said. "You've got to respect the fact that these people know the ground better than you do sometimes."

The source said that the national campaign trusts the grassroots organizers' judgment on how to split up the states in order to best canvas them.

"Don't pick petty fights over methodology with people that are working their ass off for you," the source said. "A lot of other campaigns do."

The driving force behind all the organization, traveling and campaigning is Santorum's message. His grassroots "armies" have found a number of issues that voters have identified with.

In Michigan and Ohio, voters have connected with Santorum's points on manufacturing and the "made in America" slogan; while in the South, many voters have best identified with his social issues -- specifically, they have tied together the campaign message that a strong economy and a healthy family life are correlated.

For the moment, these messages seem to have worked for Santorum as he has surged ahead of Romney in most major national polls and crept into the lead for a number of upcoming state races. Romney intends to campaign heavily in Arizona, Michigan and every Super Tuesday state, and he will likely do it while running attack ads against Santorum in virtually every one of them.

For now, Santorum's camp will likely look to stay grounded.

-- Written by Joe Deaux in New York.

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