“We realize in the political climate we live in that there are areas of our state where a Democrat is not going to win,” said Anita Gibson, the group’s president. “So we have found we need to be reaching across the aisle and making friends where we can. We’re trying to find pro-public-education candidates regardless of what political party they are affiliated with.”

Republican leaders in Alabama have complained about cross-party meddling in their primaries, but it is just as much an indication of where public policy is shaped in the region.

Southern Republican primaries have also become forums for what the party establishment sees as purity tests and what the right believes are opportunities to hold leaders accountable for their fealty, or lack thereof, to conservative principles.

Mr. McDaniel’s main campaign argument against Mr. Cochran, 76, is that the incumbent has been too willing to work with Democrats and not aggressive enough in opposing President Obama. “Mississippi is the most conservative state in the republic. It deserves the most conservative senator in the republic,” Mr. McDaniel, 41, said Saturday at a rally on the Gulf Coast.

Mr. Cochran’s brand of Republicanism predates the South’s political realignment, and his approach is more in keeping with what was the traditional bipartisan creed in the mostly impoverished region: Use longevity and influence in Washington to lure the most federal dollars to the state. “The longer I’ve been there, the more I appreciate the seniority system,” Mr. Cochran said with a smile after a rally in Jackson on Monday. “It has benefited our state in a lot of ways.”

Mr. Cochran’s argument that he should be returned to the Senate because of all the money he has brought home — with the implicit promise of more to come — is an echo of the plea that long-serving Southern Democrats once made when they faced young rivals in primaries.

In 1972, Democratic upstarts in Louisiana and Arkansas ran against senators who, like Mr. Cochran, were barons of the check-writing Appropriations Committee. The challengers, David Pryor in Arkansas and J. Bennett Johnston Jr. in Louisiana, both ran as moderates, suggesting that the conservative stalwarts were out of step with the party.