Sen. Richard Lugar’s (R-Ind.) path to reelection just got tougher, with the conservative Club for Growth PAC endorsing his primary opponent, state Treasurer Richard Mourdock.

The Club, which generally endorses against one or two GOP incumbents every election cycle, has made Lugar its first incumbent target of the 2012 election cycle.

The endorsement is a welcome piece of news for Mourdock’s campaign, which has thus far struggled to raise money. The big advantage to having the Club’s backing is its large network of wealthy donors, who have flooded previous endorsees' campaigns with hundreds of thousands of dollars.

The endorsement is also heartening to Democrats, who hope to contest the seat, but whose chances rest largely on the incumbent losing the primary. Democrats have a solid recruit in Rep. Joe Donnelly (D-Ind.), but he would face much better odds against Mourdock than against Lugar.

Senate Minority Leader Mitch McConnell of Kentucky looks on at left as Sen. Richard Lugar (R-Ind.) speaks during a news conference on Capitol Hill in November. (AP Photo/Susan Walsh)

In an op-ed Tuesday in the National Review Online, Club for Growth President Chris Chocola said the “last straw” for the Club in deciding whether to endorse Mourdock was Lugar’s continued embrace of earmarks.

“Lugar’s vote against a permanent ban on earmarks will hopefully be the capstone on a decidedly big-government, anti-economic-growth voting record,” Chocola said.

Chocola, a former congressman from Lugar’s home state of Indiana, made a point to praise the six-term senator as “a good man, and in some ways he has served Indiana and our country well,” but he said Lugar’s record just isn’t one of a conservative.

“What our country needs, now more than ever, is genuine fiscal restraint and economic growth,” Chocola said. “Lugar has time and again voted to expand the size of government, raise taxes, and increase government regulation. Examples abound through the decades.”

Lugar’s campaign and the Club for Growth PAC released dueling polls last year, with the Club poll showing Lugar tied with Mourdock and Lugar’s poll showing him leading Mourdock by 14 points.

More recently, a super PAC supporting Lugar released a poll showing Mourdock fairing much worse against Donnelly than Lugar would. While Lugar led the general election matchup 58 percent to 32 percent, Mourdock and Donnelly were tied at 42 percent.

Lugar also has the backing of Gov. Mitch Daniels (R), who is broadly popular among Indiana Republicans.

While this is the first incumbent the Club has endorsed against, it has endorsed conservative alternatives to establishment Senate candidates in Texas and Wisconsin, as well.

The big question now is whether the Club will endorse against Sen. Orrin Hatch (R-Utah) as well. Two years ago, it played a major role in unseating Hatch’s Utah colleague, Sen. Robert Bennett, at the state party convention.