The most important decision facing any presidential nominee is the choice of a running mate. It reveals not only the nominee's political judgment but also his understanding of history, and hence the critical imperative of making a wise selection.

It is a decision that potentially puts the choice literally a heartbeat away from the presidency itself. It has elevated a vice president to the Oval Office nine times, as well as positioned him to become his party's next presidential nominee on seven other occasions.

That is why the decision now facing Barack Obama demands careful consideration beyond the immediate tactical challenge of whether to offer Hillary Clinton the second spot on the Democratic ticket because she seems to want it, or because not giving it to her will alienate too many Democratic voters.

By the usual yardstick — that the choice be qualified for the presidency — she has demonstrated in the campaign just ended an impressive grasp of the major issues and qualities of leadership that attracted more than 18 million Americans to her side. Accepting her would win Obama praise on this count, while risking criticism that he had yielded to political pressure in choosing a running mate of "the past" for a campaign focused on change and the future.

More than in many past running-mate decisions, this one must factor in not only how Hillary Clinton would fit in an Obama administration. It must also weigh what challenge her husband, the former president, would pose for a young and relatively inexperienced President Obama. Based on Bill Clinton's intrusive and sometimes destructive involvement in his wife's campaign, the notion sends up an obvious red flag.

In addition, the sparks of hostility that heated the long primary campaign do not augur well for a Barack Obama-Hillary Clinton administration. Yet accepting her on the ticket would demonstrate a high degree of self-confidence on Obama's part and demonstrate his oft-repeated reliance on "the audacity of hope" that harmony could reign.

An Obama-Clinton ticket certainly would hasten the party unity that will give the Democrats their best chance to capitalize on the doldrums in which the Republicans find themselves after nearly eight years of the calamitous George W. Bush administration.

Hillary Clinton has repeatedly said, as she did on the final primary night, that she will do whatever it takes to beat John McCain in November. She proved in the later primaries how formidable she was not only with women voters but also with the male blue-collar constituency that proved elusive to Obama's message and personal appeal. And on the major issues, she has been in basic agreement with him.

If Obama is determined, however, to set a sharp new course for change, he may well feel he needs to make a clean break with the past as personified by the Clintons, and set his own course with his own people. That route would solidify his support among younger voters and help maintain the energy and enthusiasm of his remarkable campaign.

All presidential nominees vow in their search for a running mate that they are looking first of all for the individual "best qualified" to assume the presidency if destiny were to so dictate. But they usually decide on more politically practical considerations or, occasionally and incredibly, by whim.

How else, for example, can the choice by the senior George Bush of lightly regarded Dan Quayle be explained? Especially when Bush as vice president had come within a heartbeat of the presidency himself when Ronald Reagan was shot in the first weeks of their administration?

Other presidential nominees have demonstrated either crassness or desperation in their choices. Barry Goldwater in 1964 picked obscure but acid-tongued Rep. William Miller as his running mate because, Goldwater said, "He drives Lyndon Johnson nuts." And Walter Mondale in 1984 settled on low-profile Rep. Geraldine Ferraro in an obvious long-shot bid for the women's vote.

If Obama were to pick Hillary Clinton, it would be for much more than what Mondale sought. At the same time, Obama would be buying in to a more complicated bargain, which might be more than he wants or cares for.

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.