MANDEVILLE, La. — It has so far been a mostly humdrum race for Louisiana’s open United States Senate seat, likely to end in a victory for some veteran officeholder: the Republican state treasurer, one of the congressmen, maybe even, in a surprise, a Democratic public service commissioner.

But there have been gritted teeth across the state that the one candidate who has drawn by far the most attention, nationally and even internationally, is the one whom pollsters give virtually no chance, whose own party has publicly dismissed as a “hate-filled fraud” and whose unfavorability ratings approach those of North Korea’s. That candidate is a previous nine-time political contender, and eight-time also-ran, David Duke.

Mr. Duke, the former Ku Klux Klan grand wizard and open Nazi sympathizer, relishes his fame and the conundrum facing his critics, who insist on ignoring him but have been forced not to. At a time when the openly white nationalist “alt-right” has rallied to the candidacy of Donald J. Trump, Mr. Duke’s reappearance on the scene seems practically inevitable.

“After four decades, the issues that I’ve spearheaded and fought for are now mainstream,” Mr. Duke said at a seafood restaurant here, sitting across from a large, taciturn diesel mechanic turned bodyguard. Talking of what he called the egregiousness of large-scale immigration, the war on Christmas, the nefarious plotting of the “Jewish elite” and the “cultural destruction” of white America, Mr. Duke was already declaring a sort of victory on the issues: “I’ve won, in the sense that these are now mainstream.”