One of the great surprises of my first presidential campaign, back in 1960, came on the final morning of the Democratic National Convention in Los Angeles. Cecil Holland, a colleague at the old Washington Star, burst into our workspace, having discovered from his sources that Lyndon B. Johnson had just accepted John F. Kennedy’s offer of the vice presidential nomination.Barely 48 hours earlier, Kennedy and Johnson had tangled in a spirited debate before the Massachusetts and Texas delegations, continuing a fight that had started years before on the Senate floor and wound through that whole spring and summer. At times, it got really nasty, with India Edwards, a Johnson supporter, peddling the rumor (which later turned out to be true) that Kennedy was suffering from Addison’s disease.Ever since, it has always seemed the better course of wisdom not to dismiss the possibility of rivals for the presidency ending up on the same ticket. It happened again just four years ago, when John Kerry chose John Edwards as his running mate.But, with all necessary cautions, the talk of Hillary Clinton and Barack Obama winding up as partners seems truly far-fetched.I thought that was the case well before Monday, when Obama labored to shoot down the speculation while campaigning in Mississippi. He was responding to a series of comments from both Clintons. The former president speculated that together, his wife and Obama would be “an almost unstoppable force.” Earlier, Mrs. Clinton had remarked that she had heard people say of her and Obama, “‘I wish I could vote for both of you.’ Well, that might be possible someday.”Plainly irritated by the suggestion that he might need, or accept, a favor from his rival, Obama said Monday, “I don’t know how somebody who’s in second place (in delegates and popular votes) is offering the vice presidency to somebody who’s in first place.”Such vows are standard-issue rhetoric while the contest is still on. But judging from what I was told in a canvass of both the Clinton and Obama camps, there is good reason to believe this pairing will never occur. Even if the long campaign does not leave bruised personal feelings, practical considerations for both candidates argue strongly against such a deal.For Clinton, partnering with Obama, with him on top of the ticket, would either leave her part of a defeated pair in a party that is not generous with second chances or, if they won, probably lock her out of a presidential race until 2016, when she would be 68 — almost John McCain’s age now.Knowledgeable Democrats see at least two more-attractive options for her. One is to return to the Senate, where she is popular, well-established and potentially in line to be majority leader, a position with real power. The other is to go back to New York, where Eliot Spitzer’s resignation from the governorship on Wednesday leaves a potential opening for a new candidate in 2010.As for Obama, many of the same arguments apply — with even greater force. He is less enamored of the Senate than is Clinton, but it could provide a comfortable resting place, for four or eight years. Or he could go back to Illinois and run for governor in 2010, when incumbent Democrat Rod Blagojevich would face a third term.Obama would be a heavy favorite over Blagojevich or anyone else in a primary and over the nominee of the beleaguered Illinois GOP. And winning the governorship would provide the executive experience that may be the biggest gap in his current resume.But there is an even better reason for Obama to shun an offer from Hillary Clinton. With Bill Clinton poised to move back if his wife becomes president, the White House West Wing will be a miserably crowded place. There are almost always jealousies and tensions between the president’s staff and the vice president’s. But add in Bill Clinton’s ego and entourage, and serving there for Obama would really be cruel and unusual punishment.David S. Broder’s e-mail address is firstname.lastname@example.org.