By Alina Selyukh

WASHINGTON (Reuters) - Rick Santorum might be about to feel what it was like to be Newt Gingrich in Florida.

Restore Our Future, the independent "Super PAC" that supports Mitt Romney for the Republican U.S. presidential nomination, appears to be ready to turn up the heat on Santorum, Romney's suddenly surging rival.

The pro-Romney political action committee has bought about $7.7 million in TV ad time this week and the next two weeks, a period that includes a key contest in Michigan (February 28) and the run-up to "Super Tuesday" on March 6, when contests will be held in 10 states.

During the state-by-state campaign, Restore Our Future has become known largely for running attack ads on Romney's rivals - particularly Gingrich, the former U.S. House of Representatives speaker.

An onslaught of ads blasting Gingrich's ethics and leadership hammered his chances in the Iowa caucuses on January 3 and the Florida primary on January 31, Romney's biggest victory of the campaign. Restore Our Future spent more than $10 million in Florida alone.

Now, Gingrich has faded in the polls and Romney, a former Massachusetts governor, is facing a new challenge from Santorum, a former Pennsylvania senator.

Santorum, casting himself as the conservative alternative to Romney, has pulled even or gone ahead of Romney in several national polls and in voter surveys in Michigan, where Romney grew up and his father was an auto executive and governor.

If the pro-Romney political action committee continues its pattern of attacking Romney's foes, Santorum almost certainly would be the target.

Romney's campaign is separate from Restore Our Future, but Romney is in attack mode as well. During an appearance in Idaho on Friday, Romney accused Santorum of encouraging wasteful spending when he was in the Senate.

Santorum's campaign, meanwhile, released an ad this week that cast Mitt Romney as a candidate who has to rely on mud-slinging to win. In the ad, a gun-toting Romney look-alike tries to shoot a cardboard cutout of Santorum. With each shot, a blast of mud emerges from the gun.

AN ATTACK MACHINE

Super PACs, which unlike campaigns can raise and spend unlimited amounts of money, have dropped about $45 million in the Republican race to determine who will face Democratic President Barack Obama in the November 6 elections.

The race for the nomination has become a laboratory for new campaign finance rules that allow the very wealthy to shape the race for president by giving millions to PACs.

Restore Our Future does not comment on its strategy, but acknowledged it has purchased $1.8 million in ad time in Michigan, $1.9 million in Ohio, $1.1 in Tennessee and $1.1 million in Georgia.

The latter three states are among those holding contests on March 6.

Despite having the largest organization, the most money and the most high-profile endorsements in the Republican field, Romney has struggled to take control of the race.

Many conservatives do not believe he is conservative enough, and cite his evolving positions on a range of issues, from abortion rights to healthcare.

Romney's allies at Restore Our Future have delivered two new ads describing Santorum as an ultimate Washington insider with no concern for the federal deficit.

"Romney likes to do two things: Tear others down and spend gobs of cash," said Stuart Roy, an adviser to the pro-Santorum Super PAC known as the Red, White and Blue Fund.

Red, White and Blue has spent a small fraction of what the pro-Romney PAC has spent during the campaign, but the pro-Santorum group got a big boost from his victories in Minnesota, Missouri and Colorado this month.

The pro-Santorum group has reported spending about $807,000 in Michigan on TV, e-mail and direct-mail ads to voters.

"We'll be in multiple Super Tuesday states," Roy added.

Gingrich's campaign and the pro-Gingrich PAC, Winning Our Future, have taken a back seat to the Romney-Santorum duel in recent days, but that could be about to change.

Winning Our Future has been funded almost exclusively by the family of Las Vegas casino mogul Sheldon Adelson, which has given nearly $11 million to the group.

Now, the Wall Street Journal and other media are reporting that another Adelson donation, possibly amounting to $10 million, could be made before the Super Tuesday contests.

Such a donation would be huge for Gingrich, whose struggling campaign is hoping to do well enough on March 6 in several conservative states - including his home state of Georgia - to justify staying in the race.