The centrist group Main Street Advocacy on Tuesday lashed out at the conservative Club for Growth for targeting Republican incumbents in primaries, with its leader calling the Club "a cancer" on the GOP.

"The Club for Growth is a cancer on the Republican Party that prides itself on supporting rigid, divisive and obstructionist candidates," said former congressman Steven LaTourette (R-Ohio). "They are certainly entitled to support any candidate they want for any reason they want, but it would be nice if they told the voters the truth about their organization and if they won't tell voters the truth, we will."

The Club announced its first endorsement of a primary challenger last week when it backed attorney Bryan Smith against longtime Rep. Mike Simpson (R-Idaho) -- one of the more moderate Republicans in Congress and a close ally of House speaker John Boehner (R-Ohio).

LaTourette said Main Street Advocacy -- a group belonging to the former Republican Main Street Partnership -- which has to this point played a minimal role in elections, will now combat the Club "dollar for dollar."

"Members of Congress are elected by the men and women who live in their districts; their loyalty and responsibility is to those constituents not to the Club for Growth," LaTourette said. "The era of the Club for Growth getting a free pass is over. They will be held accountable."

In its release, LaTourette's group also highlighted a number of key issues on which Simpson and Club President Chris Chocola -- a former GOP member of the House from Indiana -- both voted against the Club's stated position.

Club spokesman Barney Keller said the group is fighting for the people of Idaho and noted the amount of support Simpson has received from political action committees.

“It’s a joke for Mike Simpson and his allies to cry foul on outside groups supporting his conservative challenger – 64 percent of Mike Simpson’s campaign contributions have come from Washington PACs, not the people of Idaho,” said Club for Growth Spokesman Barney Keller. "Mike Simpson is the same congressman who rakes in millions from special interests that he regulates, all while voting to raise his own pay nine times and spending thousands on lavish events at Washington D.C. social clubs.”

Republican leaders have struggled to tamp down the infighting between their incumbents and conservative outside groups like the Club and the Senate Conservatives Fund, which was formerly allied with then-Sen. Jim DeMint (R-S.C.), who now heads the conservative Heritage Foundation. But establishment and centrist groups have signaled they will try to give their incumbents some help in the 2014 election.

Earlier this year, Karl Rove helped launch the Conservative Victory Project -- an effort aimed at nominating more electable general election candidates.

GOP leaders have argued in recent years that conservative and otherwise undisciplined candidates have cost them a number of Senate seats.

Updated at 1:08 p.m.