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Re-election trouble is brewing for longtime Republican senators in deep-red states, from South Carolina to Wyoming. And the trouble is from within.

The GOP's restive Tea Party and libertarian wings, energized by their titular leader, Texas Sen. Ted Cruz, and funded in part by starve-government groups like the Club for Growth, are waging 2014 Senate primary challenges in six states — and counting.

Senate Minority Leader Mitch McConnell of Kentucky is under attack, as are Mike Enzi of Wyoming and Lindsey Graham of South Carolina. All were rated among the most conservative in the U.S. Senate last year by the American Conservative Union.

Texas Sen. John Cornyn, the Senate's second-ranking Republican, may join the list of the challenged, but so far he has no viable primary opponent.

As the fallout from the recent government shutdown and default crisis settles, here's our look at the state of play in Senate races that a Republican is likely to win. But just which Republican is the question.

Senate Minority Leader McConnell, 71, a five-term incumbent, won praise from establishment Republicans for forging a deal with Majority Leader Harry Reid, D-Nev., that ended the partial government shutdown and avoided a national debt default. He has collected endorsements from the National Right to Life Committee and former Arkansas Gov. Mike Huckabee. And he has amassed a campaign war chest of about $10 million. But his approval ratings at home are anemic, and he's fighting off a primary challenge from conservative businessman Matt Bevin, endorsed by the Senate Conservatives Fund. The fund describes itself as "a political action committee dedicated to electing true conservatives to the United States Senate."

McConnell could emerge from the primary weakened as he goes into what is shaping up to be a tough challenge from Democrat Alison Lundergan Grimes. Surveys show a potentially close general election race, with Lundergan Grimes, the secretary of state, polling particularly strongly among women.