Santorum's rise in the race has left Gingrich and Paul on the outside looking for a way in.
The former House speaker has yet to recover from a campaign nosedive that began after he won the South Carolina primary on Jan. 21, and he is pinning his hopes on his home state of Georgia to begin a comeback on March 6.
His campaign announced plans Wednesday to buy 30-minute blocks of television time in upcoming primary and caucus states for an infomercial on reducing energy prices.
Gingrich's decision not to campaign in Michigan so far has allowed Santorum to compete against Romney without also having to fend off a rival for the votes of conservatives.
Paul has yet to win any primaries or caucuses.
He has weighed in against Santorum, though, airing an ad in Michigan that challenges the former senator's claim of taking a conservative line against federal spending. The ad says Santorum voted to raise the debt limit five times, and also supported legislation that created a prescription drug benefit under Medicare.
Associated Press writers Kasie Hunt and Charles Babington contributed to this story.
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