SOMETIMES the simplest of arguments can reveal the deepest of divisions. Take the dispute between Senator Barack Obama and the Clintons over the legacy of Ronald Reagan. The episode began in Reno, Nev., you will recall, when Mr. Obama told an editorial board that Mr. Reagan was a president of ideas who had moved the country’s politics. His ideas weren’t necessarily good ideas, as Mr. Obama framed it, but they were transformative.

That may sound like a straightforward statement of history. But the Clintons pounced on Mr. Obama for presenting the icon of the red team in a positive light. Mr. Obama struck back with what has to be one of the more unusual attack ads in the history of American presidential politics. For two days, he criticized Senator Hillary Clinton on the air because she had denounced him for, as the ad put it, having expressed “a few kind words about Ronald Reagan.”

“You’re not even allowed to say a Republican had an idea,” Mr. Obama mourned on Tuesday night.

It all might have sounded like a parody of our constricted political discourse had the controversy not been so revealing of a profound split that is now quite apparent in both parties on the eve of the largest day of delegate selection ever (half of all the Democratic delegates and better than 40 percent of the Republican delegates to their respective nominating conventions will be picked on Tuesday).

Call it a split between whether politics should be a pursuit of consensus or an effort to enact a party’s fundamental ideas, its core orthodoxy.