To understand how is to understand the GOP primary.
Sen. Thad Cochran (R-Miss.) faces a stiff primary challenge from state Sen. Chris McDaniel (R). If there is one Senate candidate who's on his way to becoming the nation's tea party darling, it's McDaniel. He's supported by the anti-tax Club for Growth and the Senate Conservatives Fund, along with most other big-name conservative groups.
In the deep pool of Republican senators facing tea party primary challengers this year (half the ones running for reelection face a contested primary), Cochran appears to be the most vulnerable. He received low marks from the American Conservative Union, recently said he "doesn't know a lot about" the tea party and praised President Obama in 2008.
To tea party activists and voters with outsized influence in GOP primaries, those are unforgivable sins.
Cochran has a real race on his hands against McDaniel that neither he nor his allies are taking lightly. A super PAC has already formed to slam McDaniel and protect the senator's right flank, illustrating how much of a threat they think he poses.
But if McDaniel wins, what then? The answer is that Republicans will still be favored to win the general election, but McDaniel won't have it in the bag like Cochran likely would as the nominee.
McDaniel has stoked controversy by wavering on Hurricane Katrina relief and blaming hip hop culture for gun violence in Canada. His remarks could give pause to general election voters and open the door for Childers to compete. And McDaniel's past controversial comments could be read as a signal that he is capable of saying other things that could alienate the middle.
That's precisely what happened in 2012 in Indiana. Donnelly joined the race with only an outside chance of winning and quietly prepared for the general election as then-Sen. Richard Lugar (R) and state Treasurer Richard Mourdock (R) beat each other up. Donnelly would have had almost no chance of defeating Lugar, the more moderate candidate with a long resume in Congress. But Mourdock won and stumbled badly in the general election, making a controversial comment about rape and pregnancy that sunk his chances. Now, Donnelly is a U.S. senator.
So is Coons, who once stood little chance in the Delaware Senate race in 2010. But when tea party candidate Christie O'Donnell shocked popular and moderate Rep. Mike Castle (R-Del.) for the GOP nomination, all of a sudden Coons was back in the mix.
Childers faces a tougher climb in Mississippi, a state that is more conservative than either Indiana or Delaware. But he stands a fighting chance and has a gun-friendly, moderate profile that is about as good a fit for the general election as Democrats are going to find among candidates who have won congressional races in Mississippi. (Childers won in a very conservative -- 62 percent John McCain -- district in 2008.
In short, it's far, far too soon to crown Childers as this cycle's surprise Democratic senator. But there was time when the same could be said of Donnelly and Coons, and we saw how those stories turned out.