WASHINGTON -- An axiom. When voters watch a presumptive presidential nominee considering this or that running mate, they think: What if the president dies? When the presumptive nominee considers this or that running mate, he thinks: What if I live?

Which brings us to the dotty idea that Barack Obama should choose to have Hillary Clinton down the hall in the West Wing, nursing her disappointments, her grievances and her future presidential ambitions while her excitable husband wanders in the wings of America's political theater with his increasingly Vesuvian temper, his proclivity for verbal fender benders and his interesting business associates. That this idea survived her off-putting speech Tuesday night, after Obama won the right to choose a running mate, is evidence that many Democrats do not fathom the gratitude that less-blinkered Americans feel for Obama because he has closed the Clinton parenthesis in our presidential history.

After some of the boilerplate geographic pitter-patter that today's candidates consider Periclean eloquence (" ... from the hills of New Hampshire to the hollows of West Virginia ... "), she obliquely but clearly identified herself as the person who would be "the strongest candidate and the strongest president" and, pointedly, the person most ready to "take charge as commander in chief." There is a fine line between admirable tenacity and delusional denial, and Clinton tiptoed across it.

Obama's choice of a running mate will be the first important decision he makes with the whole country watching, so it will be a momentous act of self-definition. If he chooses her, it will be an act of self-diminishment, especially now that some of her acolytes are aggressively suggesting that some unwritten rule of American politics stipulates that anyone who finishes a strong second in the nomination contest is entitled to second place on the ticket.

Behind the idea that Obama should run in harness with Clinton is this wobbly theory: Because the Republican Party is in such bad odor, if you unify the Democratic Party, that will suffice to win the election, and she is a necessary and sufficient catalyst of unity. But she is neither. She would be a potent unifier of John McCain's party, thereby setting the stage for exactly what the nation does not need, another angry campaign of mere mobilization rather than persuasion.

Surely she, the most polarizing Democrat, is not the only Democrat who can help Obama appeal to the voters who rejected him in Kentucky and West Virginia. And as his running mate, she would nullify his narrative. The candidate embracing the "future" should not glue himself to Washington circa 1993. Someone promising to "turn the page" should not revert to an earlier chapter. Someone whose mantra is "change" should not embrace her theme of restoration -- that the 1990s were paradise and Democrats promise paradise regained.

She, whose experiences as First Spouse have not impressed Obama as acquisitions of national security expertise, would not help him deflect McCain's predictable attack on his thin curriculum vitae. And the more she seems to be pushing Obama to choose her, the more resolutely he must resist. Otherwise, at the beginning of a contest in which McCain will portray him as a flimsy figure, Obama will define himself as someone who can be pushed around.

On the eve of the battle of Trafalgar, Admiral Nelson, addressing his captains on the HMS Victory, picked up a fire poker and said: It does not matter where I put this -- unless Bonaparte tells me to put it a particular place. Then I must put it someplace else. Is Obama Nelsonian?

Selecting vice presidential candidates has recently become more serious than it was when Richard Johnson became Martin Van Buren's running mate in 1836 partly on the strength of the slogan "rumpsey dumpsey, rumpsey dumpsey, Colonel Johnson killed Tecumseh," a distillation of the unsubstantiated story that he personally killed the Shawnee chief at the Battle of the Thames in the War of 1812.

In 1964, Barry Goldwater did not reassure queasy voters when he said that one reason he chose to run with Bill Miller, an obscure upstate New York congressman, was that Miller annoyed Lyndon Johnson. And remember the frivolousness that produced Maryland Gov. Spiro Agnew as Richard Nixon's running mate in 1968 and Missouri Sen. Tom Eagleton as George McGovern's in 1972.

Clinton, having risen politically in her husband's orbit, is a moon shining with reflected light. Were Obama to hitch himself to her, he would reduce himself to a reflection of a reflection.