By Alina Selyukh
WASHINGTON (Reuters) - In the wild race for the Republican presidential nomination, there has been one constant: When Mitt Romney really needs to do well in a state, he and his allies pour money into ads, most of which attack his rivals.
Romney's big-spending tactics battered Newt Gingrich into a fourth-place finish in Iowa, and helped Romney win key contests in New Hampshire, Florida, Nevada, Michigan and Ohio.
Now, the former Massachusetts governor and his allies have invested nearly $5 million on the next big prize in the state-by-state race for the Republican nomination: Illinois, which will hold its primary on Tuesday. Most of the spending will go toward ads that attack Rick Santorum, now Romney's chief rival.
Romney's campaign, which spent nearly $19 million in January, has spent almost $1 million on cable and broadcast ads mostly in the Chicago area, according to a Republican media buyer who also tracks campaign advertising.
It appears Illinois' primary also could become the latest example of the dramatic impact that independent "Super PACs" are having on the 2012 campaign.
Restore Our Future, a political action committee that supports Romney and has spent well over $30 million on campaign ads - largely attacking Gingrich or Santorum - has reported spending $3.7 million on anti-Santorum ads that have aired mostly in the Chicago area, the media buyer said.
Santorum and Gingrich, who have battled to be the more conservative alternative to Romney, have lagged far behind him in fundraising.
At the end of February, Romney's campaign said it had raised nearly $75 million. Santorum's indicated it had raised about $16 million. Gingrich's campaign has not revealed its February haul, but it had raised just more than $18 million by the end of January, according to reports filed with the Federal Election Commission.
Campaigns are limited to donations of $2,500 from individuals during the primary season, and another $2,500 for the November 6 election, when the Republican nominee will face Democratic President Barack Obama.
Super PACS, on the other hand, can receive unlimited donations. During the nomination battle, a few very wealthy donors have propped up PACs supporting Gingrich and Santorum, allowing those candidates to fire back when they came under attack from Romney and his allies.
Winning Our Future, a pro-Gingrich PAC, raised $13.1 million through the end of January - including nearly $11 million from the family of Sheldon Adelson, a Las Vegas casino mogul.
Adelson appears to have donated an unspecified amount in February, but with Gingrich fading in the race, it is unclear whether Adelson will be willing to continue the financial support.
The Red, White and Blue Fund, a pro-Santorum PAC, raised $2.8 million through the end of January - including $2 million from Wyoming investor Foster Friess and Global Industries founder William Dore.
A PRIORITY FOR ROMNEY
Illinois, Obama's home state, is more politically moderate than Southern states where Romney has struggled to attract support from conservatives. The state is also likely to be key in the November election.
That has helped make it a priority for Romney, who held a slim lead over Santorum, a former U.S. senator from Pennsylvania, in a recent Chicago Tribune/WGN poll of Illinois voters.
None of the other campaigns has had a presence on Illinois TV screens yet.
Winning Our Future, the pro-Gingrich PAC, has spent about $17,600 on Internet and radio ads there.
The pro-Santorum Red, White and Blue Fund is weighing its options for Illinois, said Stuart Roy, who advises the group. He said the PAC had bought $260,000 worth of ads in Louisiana, a conservative state in the South that votes on March 24.
That is in addition to about $30,000 in cable TV ads that Santorum's campaign has purchased in Louisiana, according to the Republican media buyer.
"The challenge (in Illinois) is it's an expensive state (for advertising), but I expect that (Santorum) not only has a chance to be competitive there but he will be very competitive," Roy said.
Romney's Super PAC allies have invested about $537,000 in TV and direct-mail ads in Louisiana. The total is likely to rise because most of the ads ran through the early part of this week, the media buyer said.
Gingrich's campaign has not had much of a presence on the airwaves recently, a sign the former U.S. House of Representatives speaker's effort could be running low on money.
(Additional reporting by Alexander Cohen; Editing by David Lindsey; Desking by Peter Cooney)