If he continues to serve as Senator from Illinois (or, as rumored, runs at some point for governorship in his home state in order to gain executive experience) he’ll be able to provide an independent, compelling voice on the issues. He could support the president or oppose the president, without any expectation of loyal, dutiful, automatic agreement. If a Clinton presidency founders, he’d be perfectly positioned to offer a competing vision.

Moreover, there’s always the possibility that a Clinton-Obama pairing loses in November – in which case his presence on the ticket would do him more damage than good. John Edwards’ campaign this year demonstrated the way that a failed race for the Vice Presidency gives little advantage in future campaigns.

In short, a Vice Presidential nomination offers Obama no substantive benefits while depriving him of some of his independence and freedom of maneuver.

He’d never again be able to run as plausibly as a candidate of “change” if he serves eight years as Hillary’s Veep. Inevitably, he would become a candidate associated with the infamous Clinton Machine, and tainted by all its corruptions and compromises. Obama knows history, and remembers the way that distinguished, powerful Senators of the past like Hubert Humphrey and Al Gore took the Vice Presidency and then found their own runs for the White House shadowed by the controversial legacies of their bosses (LBJ and Bill Clinton)

And worst of all for Barack, the magical aura of this campaign, with huge crowds intoxicated at the very idea of a black president, couldn’t be recaptured if he’s already run in the number two position. If we’ve already elected (or even nominated) an African-America as Vice President, the idea of a future Presidential race wouldn’t be nearly as novel or thrilling.

Consider here the example of Joe Lieberman: his nomination as Vice President in 2000 provoked a wave of deep emotion and exultation in the Jewish community. When the same guy ran for the presidency just four years later, the whole idea seemed anti-climactic, unimportant, and produced little excitement in the Jewish community or anywhere else. The barrier had already been broken with the Vice Presidential campaign and the same candidate doesn’t get credit for breaking it twice.

Why Not Obama-Clinton?

With all the potent reasons for Hillary to disregard the idea of running with Barack, and the even more decisive reasons for him to turn her down even if she offered, some Democrats love to consider the upside-down arrangement: why not an Obama-Clinton ticket?

If anything, this notion looks even less plausible.

Those who know Hillary report that she thoroughly enjoys her well-established role in the U.S. Senate. If she lost to Obama in the Presidential race, she’d greatly prefer to return to Capitol Hill—where she could probably serve for the rest of her life and stands an excellent chance of succeeding Harry Reid as Majority Leader.

As a prominent part of the previous Clinton administration, she witnessed first hand all the indignities and burdens thrust upon Vice President Al Gore—in fact, she administered some of them herself. Why would she want to open herself to the humiliation of waiting around in a powerless, second-string job in case some disaster befell a conspicuously young and vigorous president?

Hillary would also find it difficult to use the Vice Presidency as a stepping stone to a future Presidential run. She’d be 68 at the end of a second Obama term, and the generational shift his nomination signaled would make it most unlikely that Dems turn backward to a Baby Boomer restoration with an aging Hillary.

As for Obama, he wouldn’t need her and wouldn’t want her on his ticket. If he beats her in the fight for the nomination this year, he’d want to complete the Democratic disassociation from the nasty politics of the Clinton era. Barack’s insistence on “change” and a “fresh” approach signifies not only a departure from the age of Bush, but also closing the chapter on the age of Clintons. Hillary on the ticket inevitably brings Bill with her, as a third figure at the top of the party, and undermines the idea of a generational changing of the guard and the adoption of a new, more unifying tone.

Finally, no President wants the malevolent combination of Bill and Hill scheming against him from a new vantage point in the Vice President’s residence at the Naval Observatory.

If Obama feels that the bruises of the campaign require a female running mate he could find any number of preferable alternatives – particularly Governor Janet Napolitano of Arizona. She has the executive experience that both Obama and Clinton lack, as a former US Attorney, state Attorney General, and Democratic governor in a Republican state who won re-election by a nearly two-to-one margin. She’s also won a reputation as moderately tough on illegal immigration, and might even help Obama compete with John McCain in his home state. Best of all, Napolitano (or other Democratic governors like Kathleen Sebelius of Kansas, or Senators like Dianne Feinstein of California) come without the heavy baggage of the Clintons’ soap-opera marriage and reputation for political thuggery.

Concerning the much-discussed “dream ticket,” most observers concede that neither Obama nor Clinton want it. When you combine their joint reluctance with the fact that neither candidate would truly need the other in order to run effectively against the Republicans, the likelihood of the uncomfortable alliance becomes very remote indeed.