INDIANAPOLIS (AP) — The U.S. Chamber of Commerce became the latest national interest group to stake a claim in Indiana’s heated Republican Senate primary, announcing Tuesday it is endorsing U.S. Sen. Richard Lugar in his toughest re-election battle in decades.

The national business organization joined local business groups, including the Indiana Chamber of Commerce and Indiana Manufacturers Association, who announced their support for Lugar last week.

Conservative groups, including the National Rifle Association and the anti-tax Club for Growth, have lined up behind Indiana Treasurer Richard Mourdock, who is challenging Lugar in the May 8 Republican primary.

The endorsements show a split between the old guard and more ideological groups, said Margaret Ferguson, who heads the political science department at Indiana University-Purdue University Indianapolis.

Historically, major interest groups like the chamber have given money to incumbents based on the assumption they would win re-election, she said.

“The chamber is kind of an old-school conservative, mainline conservative group,” she said. “Giving money to a candidate that doesn’t win doesn’t do you any good whatsoever.”

Lugar is facing his toughest re-election battle since 1982 in large part because of tea party resentment over votes he’s taken in the Senate, but also because of a series of political stumbles that have played into opponents arguments that he is out of touch with Hoosiers.

US Chamber spokeswoman Blair Latoff called Lugar a “long-time ally” of local Indiana businesses. She would not say how much the chamber would spend supporting Lugar, however they already sunk $238,000 into ads supporting Lugar’s push for construction of the Keystone XL oil pipeline, which would bring oil from Canada into the U.S.

A series of polls have shown Mourdock within striking distance of Lugar with less than three weeks before the primary. The closeness of the battle has spurred outside groups to flood Indiana’s airwaves with ads attacking both candidates.

A handful of groups have gone well beyond nominal endorsements and put extensive money behind Mourdock’s campaign. Club for Growth has far outspent other groups, buying close to $1.25 million in airtime as of Tuesday.

Posted 4/18/2012