Barack Obama's choice of Joe Biden to be his running mate has generated almost as much chatter about why it wasn't Hillary Clinton as about why it was Biden.

Obama's first stated reason for picking the 36-year Senate veteran and chairman of the Senate Foreign Relations Committee was that he was well qualified to be president if destiny so dictated. But pretty much the same could have been said about Clinton, for whom an astounding 18 million Americans voted in the primaries.

An obvious second reason for selecting Biden is that he shores up the ticket's resume on foreign policy experience. But Clinton's huge backing among older women would similarly have bolstered Obama with such female voters.

Biden was clearly more compatible with Obama than Clinton, as Obama looked down the road to a presidency in which, as he put it in introducing Biden, he was planning a governing "partnership" with his vice president.

Quite aside from all the competition of the Democratic nomination fight that ultimately sharpened personal edges between Obama and Clinton, there was the matter of his desire if elected to run his own presidency and chart his own course.

Biden, at 65 and after two failed presidential bids, was ready and eager to play the traditional vice-presidential campaign role, not just in taking the fight to John McCain but also in comfortably accepting the traditional second-banana function in an Obama administration.

Clinton, at 60, for all her earnest support for Obama's election as a team player after losing the presidential nomination, still has reason to keep her eye on the Oval Office four or eight years hence. For her much more than for Biden, the vice presidency would have been, as it has been for many others, a steppingstone toward a future presidency.

The increased power and influence of the vice presidency in the hands of Dick Cheney has graphically demonstrated how the office can become a parallel if not a rival power center. It's true that it has happened with the approval or at least acquiescence of President Bush, and Obama would not likely have let the same thing occur with Clinton as his vice president.

But there was also the matter of former President Bill Clinton, and the potentially unsettling prospect of having him rattling around the East Wing of the White House as a super-kibitzer, with remnants of the sour-loser side of him that he flashed during the primaries and immediately thereafter.

No, these clouds over a Hillary Clinton vice presidency were too depressing to contemplate for a presidential nominee who had just beaten the party establishment candidate. Obama needed foremost to remain the inspirational leader that had gotten him to where he now was.

Much is being made by critics of the Biden choice that the perpetually grinning and upbeat Delaware senator will be a high-risk candidate in the traditional attack-dog mode, given as he has often been to shooting from the hip and running off interminably at the mouth.

But Biden showed in the multiple Democratic primary debates that he could keep his rhetoric and logorrhea in check. Probably his most memorable debate sound bite was, when asked directly if he could muzzle himself, he replied with tight lips and a straight face "Yes." As the audience roared, he broke into a broad grin.

The Republicans obviously will resurrect some of Biden's most damaging baggage from his failed 1988 campaign, including examples of plagiarism in speeches and exaggerations of his academic grades. But this is mild stuff compared to the sex scandals and corruption that have plagued political figures in recent years.

Back in 1980, Republican presidential nominee Ronald Reagan toyed at the party's convention in Detroit with taking former President Gerald Ford as his running mate. In a television interview, the venerable Walter Cronkite asked Ford whether he was thinking of "something like a co-presidency." Ford didn't use the term himself, but replied: "That's something Governor Reagan really ought to consider." It was enough to scare Reagan off.

Obama this time around clearly wanted no part of a "co-presidency," so picking Biden over Clinton was a no-brainer.

Jules Witcover's latest book, on the Nixon-Agnew relationship, "Very Strange Bedfellows," has just been published by Public Affairs Press. Reach him at juleswitcover@earthlink.net.