But I'm not sure it's ever been true. I remember how Ronald Reagan cut up Gerald Ford in 1976 and I never thought it helped Ford all that much.
Nor did I ever believe Jimmy Carter was helped when Ted Kennedy challenged him in 1980.
Whatever the case, neither Ford nor Carter won after those primary challenges. Carter bested Ford in '76, then lost to Reagan in '80.
And this year? Sure, Al Gore got himself a new wardrobe and overhauled his campaign staff. I give him good marks for that, just as George Bush gets high marks for raising so much money so fast.
But I think both of them come away from the primaries as weaker candidates.
When Bill Bradley brought out that Gore didn't always give straight answers about his past voting record, it raised questions about Gore's veracity. That's not a good thing when you're Bill Clinton's vice president and you're trying to explain to voters that you're not like Bill Clinton.
Bush fared even worse. He began as the good natured candidate whose bipartisan approach to being governor of Texas promised a Big Tent philosophy that would bring new voters into the Republican Party.
But then his campaign turned right and there he was courting the religious right and complaining about Democrats voting in Republican primaries.
And, when his friends secretly launched sleazy ads attacking McCain, Bush said the ads weren't helping him, but he couldn't do anything about them.
Taken at his word, that suggests he has lost control of his own partisans, hardly a plus.
It's still a long time until November, but both candidates have picked up some heavy baggage and how they handle it will go a long way toward determing which of them will eventually become president.