To many conservatives, what happened this week in Mississippi’s U.S. Senate race amounted to theft. I disagree. Instead, it offered a reassuring sign of flexibility in an American political system that has given us all cause to worry.

In that race, Chris McDaniel, a tea party extremist, was widely expected to win the Republican primary against longtime GOP incumbent Thad Cochran and thus win the seat. McDaniel had the momentum, and he had the support of a majority of the state’s conservative Republicans.

So the Cochran camp decided to think outside the box. The incumbent senator all but begged Mississippi Democrats to cross over and cast votes for him in the GOP primary, warning that a McDaniel victory might mean significant cuts in social programs and farm subsidies relied upon by millions of Mississippians. He offered Democrats an alliance, and tens of thousands accepted that alliance.

But it goes even deeper than that. In fact, it goes bone deep. In Mississippi, most Democratic voters are black voters, and by inviting large numbers of black voters into the sanctuary of a Republican primary, Cochran violated a taboo that over decades has lodged itself deep in the Southern subconscious, unspoken and unrecognized.

It wasn’t always unspoken. In fact, its chief architect and proponent was Henry W. Grady, the famous 19th century editor of this newspaper. He believed in white superiority, and he argued vehemently that the only way to sustain that superiority was for whites to band together in one dominant party. Within that party, they could work out any differences among themselves, white people to white people.

“The very worst thing that could happen to the South is to have her white vote divided into factions, and each faction bidding for the Negro who holds the balance of power,” Grady warned. If you allowed black voters to play a role, they would start demanding things like equality — “race privileges,” Grady called them — as well as a share of the economic spoils. And then, all hell would break lose.

For almost a century, Grady’s strategy dictated the course and form of Southern politics. For a long time, it was enforced through the all-white Democratic Party with its white-only primaries. Such primaries were outlawed by the Supreme Court in the 1940s, but by then, the taboo no longer needed legal enforcement. It had become instinct and culture.

That deeply ingrained cultural norm explains why the Democratic Party of the ’70s and ’80s could not last as a coalition of white and black. It also explains how the white Southern power structure so quickly reassembled itself, ironically, under the Republican banner of Abraham Lincoln. Even now, you still hear its echo — for example, in McDaniel’s defiant refusal to admit defeat.

“Before this race ends,” he said, “we have to be absolutely certain that the Republican primary was won by Republican voters,” although the law requires no such thing.

These days, the taboo may be driven less by active racism than by deep cultural memory, but it exists, and it still does damage. It undoubtedly animates a portion of the outrage against Cochran. He won by breaking a rule in force for a long, long time, but it needed breaking.