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The ads both attack the Democratic candidates in each state — former U.S. Surgeon General Richard Carmona in Arizona and Rep. Joe DonnellyJoe DonnellyIndiana GOP rep: Likely primary opponent 'lying about my family' Dem senator to sell stock in family company that uses outsourced labor Vulnerable senators raise big money ahead of 2018 MORE in Indiana — for what they characterize as the candidates' comfort with raising taxes and support for President Obama's healthcare reform law.

But they also each assert that supporting the Democratic candidate would mean "a U.S. Senate controlled by liberals," as the ad running in Indiana states.

"Do you know that a vote for Carmona is a vote to put liberals in charge of the U.S. Senate?" the Arizona ad asks.

It's an interesting tactic, as the Senate currently has a Democratic majority. But it also might work in red-leaning states like Indiana and Arizona, where Mitt Romney has led Obama by double digits in a number of polls.

The Club for Growth boosted both Rep. Jeff FlakeJeff FlakeSen. Flake's GOP challenger: McCain should resign Senators who have felt McCain's wrath talk of their respect for him Arizona senator: McCain still focused on healthcare legislation MORE (R-Ariz.) and Indiana State Treasurer Richard Mourdock (R) in their primaries with endorsements and funds, and both are running tight races against their Democratic challengers in states Republicans believe could help them regain a majority in the Senate.