The founder of the Club for Growth's Pennsylvania chapter on Tuesday called for Pat ToomeyPatrick (Pat) Joseph ToomeyTop GOP candidate drops out of Ohio Senate race Newly declassified memos detail extent of improper Obama-era NSA spying Overnight Tech: FCC won't fine Colbert over Trump joke | Trump budget slashes science funding | Net neutrality comment period opens MORE, the national group's former president, to withdrawal from the 2010 Pennsylvania Senate GOP primary because Toomey cannot win a general election against a Democrat.

Toomey is challenging Sen. Arlen Specter (R) for the second time in 2010.

"Today I am urging Pat to withdraw from the 2010 Senate race because his candidacy threatens to hand over total control of the federal government to the Democrats," William Parker, Club for Growth's founder, wrote in a letter.

Parker endorsed Toomey in his 2004 run, which came two points short of unseating Specter. But in 2010, Parker said Specter is the "only person who can keep that Senate seat in the GOP column."

It is worth noting that the Club for Growth, under new President Chris Chocola, has already endorsed Toomey's Senate campaign.

Parker compared Toomey to former Sen. Rick Santorum, a Republican who lost his bid for re-election in 2006. Toomey is to the right of Santorum, Parker reasons, so he won't be successful in Pennsylvania.

"Pat can't win the general election," Parker wrote. "His lifetime American Conservative Union record of 97 puts him farther to the right than Rick Santorum's 88 percent lifetime rating. Rick was a two-term Senator, ranked number three in the Republican leadership, spent $31 million and lost by 18 points."