For all of the Tea Party’s struggles this year, it has a real chance to unseat an incumbent Republican in the Mississippi Republican primary Tuesday. Thad Cochran, the first Republican senator from Mississippi since Reconstruction, is facing a strong challenge from Chris McDaniel, a Tea Party-backed state senator.

The most recent — if imperfect — polls show a tight race, even though Mr. McDaniel’s campaign has been dogged by an unusual scandal: Four McDaniel supporters were arrested in a case in which one of them was accused of entering the nursing home where Mr. Cochran’s wife is being treated for dementia and of taking pictures of her.

A statewide Tea Party-versus-establishment fight might seem new in Mississippi, but it is only the most recent duel in an ancient struggle between the Southern establishment and populist, anti-establishment voters.