GOP rival gets support after Sen. Lugar backs debt bill

Updated at 3 p.m. ET

A leading conservative group is getting behind the GOP primary opponent of veteran Sen. Richard Lugar, a day after the Indiana senator voted for the bill to increase the debt limit.

Citizens United PAC is endorsing Indiana Treasurer Richard Mourdock, who lags Lugar in fundraising. The endorsement comes with $10,000 to help Mourdock in the 2012 primary and general election, according to Politico.

Citizens United, led by David Bossie, gained national attention as the plaintiff in a Supreme Court ruling that opened the door to unlimited political donations from corporations and unions.

Lugar, the Senate's most senior Republican, is in the cross hairs of fiscal conservatives such as the Club for Growth as he seeks a seventh term. He was one of 28 Republican senators to support the bill, which raises the debt limit and cuts spending by $2.1 trillion over the next decade.

INTERACTIVE:  Senate vote

In July, the Club for Growth ran ads warning Lugar to vote against an increase in the debt ceiling.

GOP senators in the Senate Tea Party Caucus, including co-founders Jim DeMint of South Carolina, Rand Paul of Kentucky and Mike Lee of Utah, were among the "no" votes.

Lugar touted his efforts to reduce the debt, including support for a "cut, cap and balance" bill backed by many conservatives, in a news release Tuesday.

"Though the bill isn't everything we wanted, it's still a victory for conservatives over President Obama's out-of-control spending and big government policies. And we stopped his efforts to increase taxes dead in its tracks," he said.

A recent poll for the Club for Growth found Lugar and Mourdock essentially tied for the GOP Senate nomination. Lugar's own internal poll found he has a double-digit lead over his challenger.

Note: This story was updated to correctly reflect the Club for Growth's role.