ABC News' Michael Falcone reports:

MANCHESTER, N.H. - Though there's still a primary being fought in New Hampshire, the Republican presidential candidates and independent groups supporting them are already jockeying for television air time in the Palmetto State.

Mitt Romney released a new T.V. ad on Thursday accusing the Obama administration of trying to "play politics with jobs in South Carolina."

A GOP source who is tracking ad buys in early primary states told ABC News that the Romney campaign has spent a grand total of $551,000 on ads in South Carolina so far this election cycle, $230,000 of which is paying for air time for the new spot.

"The National Labor Relations Board, now stacked with union stooges selected by the President, says to a free enterprise like Boeing, 'You can't build a factory in South Carolina because South Carolina is a Right to Work state,'" Romney says in the ad, referring to a the NLRB's legal action against the airplane manufacturer.

Against the backdrop of images of a Boeing plane, Romney adds, "That is simply un-American."

The ad will air on broadcast stations in Charleston, Columbia, Greenville, Spartanburg, Myrtle Beach and Florence until Jan. 7 and will be shown on cable until the Jan. 21 - the date of the state's "first in the South" primary. The campaign is likely to increase its buy in the coming days too. The Romney campaign began buying up airtime on South Carolina cable in mid-December, but their latest purchase of airtime is the first time their ads will appear on local broadcast channels.

And Romney is hardly the only candidate getting into the action. Rick Santorum, Ron Paul and Rick Perry have all made fresh advertising buys in the state. With a Romney win in New Hampshire looking all but inevitable based on recent polling, South Carolina could be a last-stand state for several of the other GOP contenders.

Romney's closest competitor there, former Sen. Rick Santorum, is also making an aggressive push on the airwaves. ABC News has reported that the Santorum campaign is planning a major buy on both cable and broadcast stations. But the campaign has a lot of catching up to do compared to Romney. To date, Santorum has only spent $81,000 on television ads in the Palmetto State.

His campaign is also getting a boost from a pro-Santorum super PAC, the Red, White and Blue Fund, which is placing a $190,000 ad buy in the Greenville, Charleston and Columbia markets.

A new CNN-Time Magazine-Opinion Research Corporation Poll of South Carolina released on Friday shows Romney leading Santorum, 37 percent to 19 percent. Gingrich is close behind in third place with 18 percent, followed by Paul at 12 percent, Perry at 5 percent and Huntsman at 1 percent.

Both Paul and Perry are trying to boost those numbers by purchasing more airtime. Perry, who has decided not to compete actively in New Hampshire, has spent a grand total of $220,000 on television spots and is booked on the air from Friday through Jan. 12. The Perry campaign is  deploying an already-produced 30-second TV spot titled "Values," which highlights Perry's small town Texas upbringing.

Paul is investing even more in South Carolina. His campaign has booked $238,000 of ad time on broadcast and cable stations in from Jan. 9 through Jan. 15. So far, the Paul campaign has spent a total of $457,000 in the state. One of his campaign's ads includes a stinging criticism of Santorum, calling him "another serial hypocrite who can't be trusted."

And a pro-Paul group called the Santa Rita super PAC, has booked $139,000 of cable time from Jan. 9 until the day of the primary.

But aware of just how significant a decisive victory could be in South Carolina, the Romney campaign is upping the ante by releasing a radio ad echoing the business-minded message of its television commercial. Romney is spending $22,000 to air the ads on radio stations in the Charleston, Columbia, Greenville-Spartanburg and Florence-Myrtle Beach markets.