* Romney and allies continue to far outspend Santorum
* Santorum, Romney Super PACs build up Ohio presence
* Pro-Gingrich Super PAC returns after new donation
WASHINGTON, March 1 (Reuters) - Even as Republican front-runner Mitt Romney's opponents nip at his heels in the state-by-state march toward the party nomination, he and his "Super PAC" allies remain comfortably ahead in their ability to outspend rival presidential hopefuls.
The advantage is now showcased in Ohio - the biggest prize in next week's "Super Tuesday" showdown when 10 states vote for the Republican party nominee - where Romney and his supporters are outspending main rival Rick Santorum and his backers in TV advertising by a margin of almost four-to-one.
The pro-Romney Super PAC, Restore Our Future, has spent $2.2 million on ads in Ohio, alongside $1.3 million laid out by the Romney campaign itself on TV and cable spots in the state, according to the Super PAC's spokeswoman and a Republican media buyer who tracks campaign spending.
The pro-Santorum Red, White and Blue Fund on Thursday doubled its Ohio TV spending to $514,000, according to adviser Stuart Roy. The Santorum campaign itself has invested some $410,000 in cable ads there, according to the buyer's data.
Super PACs are fundraising organizations that operate independently from individual candidate's campaigns and can raise unlimited amounts from individuals, corporations and unions and spend to support candidates or issues.
Big spending on negative ads against his Republican opponents has kept former Romney, a former Massachusetts governor, ahead of the pack for most of the nomination race. His financial clout was most notable in the Jan. 31 primary in Florida where the he and his Super PAC spent heavily on negative ads to defeat Newt Gingrich.
Restore Our Future is edging toward $6 million in spending on TV ads in four of the 10 "Super Tuesday" states: Ohio, Tennessee, Oklahoma and Georgia. Most of it is aimed at attacking Santorum with spots painting him as a Washington insider with a big-spending voting record from his time as a Pennsylvania senator.
A new Restore Our Future ad out on Thursday claims Santorum is not so conservative as he makes out to be.
Polls show the two men running a close race in Ohio, a critical swing state in November's general election against President Barack Obama.
"Ohio is viewed as, if you can win Ohio then you can point to it and beat your chest and say I can win in the fall," said Terry Casey, a Republican analyst in Columbus, the state's capital.
SANTORUM TRIES TO CATCH UP
Santorum's fund goes after Romney as well as Gingrich in one of the ads for lacking the consistent conservative values and economic plans to be able to tackle Obama.
Despite being way behind in spending on TV ads in Ohio, Santorum and his backers have spent slightly more there on other forms of advertising.
They have spend about $500,000 on direct mail and phone calls to voters, compared to just under $200,000 on Internet and direct mail advertising by Restore Our Future.
Helping bridge the Ohio spending gap slightly is a conservative anti-abortion group Susan B. Anthony List that is airing pro-Santorum ads on Ohio radio stations.
Similar to what they did in Michigan, where Santorum lost to Romney by a slim margin in Tuesday's primary, the non-profit group bought $200,000 worth of radio ads in Ohio, where its leaders are now on a bus tour.
The group's originally heavily social message of Santorum's conservative views of family and marriage is now sprinkled with the reference to the candidate's economic plan, a paramount issue to voters in Ohio, which has not regained the jobs lost during the recession.
"(Santorum) is innovative in explaining that a foundation of strong families is part of building a strong economy," said Maggie Gallagher, president of the Institute for Marriage and Public Policy who is part of the bus tour.
The pro-Santorum PAC has yet to expand beyond Ohio, but is eyeing Tennessee and Oklahoma, where Santorum is edging Romney in the polls. The group has invested $230,000 in direct mail and phone calls to voters in Tennessee and the less high-profile states of North Dakota, Alaska, Idaho - all Super Tuesday states - and Washington, which votes on Saturday, March 3.
Libertarian Ron Paul's campaign is the only challenger to Santorum on the airwaves of Washington and Idaho, with an ad called "Three of a Kind" hitting all of Paul's rivals for policy inconsistency.
The Gingrich campaign and his Super PAC had been on a weeks-long hiatus from advertising, apparently running low on cash. His PAC, Winning Our Future, has now been resurrected, thanks to its single largest donor, Las Vegas casino mogul Sheldon Adelson.
Following news that he wrote a new check to the PAC, Winning Our Future has now reported spending nearly $3 million on advertising in the very same four "Super Tuesday" states of Ohio, Tennessee, Oklahoma and Gingrich's home state of Georgia, where the stakes for him are highest.
The group on Thursday also reported buying $1.1 worth of ads in Kansas, which votes on March 10, and Mississippi and Alabama, which vote on March 13. (Additional reporting by Alexander Cohen; Editing by Philip Barbara)