With his nomination in hand but his party divided, Barack Obama needs a running mate who can help unify the Democrats — and help them win in November.

It would make sense for him to pick someone who backed Hillary Clinton, whose supporters will comprise nearly half the convention delegates in Denver. The harder part is who. Clinton backers who figure in the speculation include Sen. Evan Bayh of Indiana, former Gov. Tom Vilsack of Iowa, Govs. Ted Strickland of Ohio and Ed Rendell of Pennsylvania, and Gen. Wesley Clark.

But the best way for Obama to ensure unity — and victory — is to pick Clinton, who signaled this week that she'd certainly consider the vice presidential slot.

Despite causing awkward moments — her comment about Obama's weakness with "white Americans," her clumsy reference to Robert F. Kennedy's assassination, her grudging acceptance of defeat — Clinton is the one potential running mate who would clearly boost his chances.

She has been a terrific campaigner and is especially strong with members of core Democratic groups that have been cool to the Illinois senator: white women, Hispanics and blue-collar workers. Obama and Clinton also would be ideologically compatible, with similar positions on major issues.

Whether they're personally compatible is more problematic. But two words answer those who say campaign animosity precludes such a ticket: Lyndon Johnson.

Indeed, the 1960 race produced even greater personal bitterness between Johnson and John F. Kennedy than this year's Obama-Clinton race. Johnson supporters accused Kennedy of covering up the state of his health, accurately as it turned out. Johnson bashed Kennedy's father.

But when the crunch came, Kennedy picked Johnson as the strongest running mate. Johnson accepted, in part, because he thought he would have more clout as vice president than as Senate majority leader.

Besides, he knew that, other than winning the presidency, the best way to reach the White House was to serve as vice president. Today, 14 of the 43 presidents had been vice president, seven in the 20th century alone.

That may be why Clinton is hinting her interest, given that a Democratic victory is more likely than not. At 60, her only other hope of reaching the White House would be if Obama lost in November.

For Obama, the key argument is that she adds more than any other possible running mate, many of whom would be untested under fire. A recent Fox News poll showed that an Obama-Clinton ticket would beat a John McCain-Mitt Romney ticket, but that Obama, alone, would lose to McCain. A Michigan survey showed the same result in that state.

Yet there are barriers to an Obama-Clinton ticket. Here are some questions:

Would picking a veteran Washington figure damage Obama's "change" message? Or would choosing a woman to run with a black enhance the extent that his candidacy does represent change?

Would it hurt to have no white male on the ticket? Perhaps. But Democrats won't win a majority of white male voters anyway. Spurring turnout among minorities and women could more than offset any lost votes from white men.

What about Bill Clinton? Probably the biggest barrier, given that the former president continues to appear somewhat out of control, most recently in his heated response to a Vanity Fair article about his personal and financial baggage.

Still, he remains popular, especially among Hispanics, in Southern states like Arkansas and in small towns. Remember that old Clinton slogan about getting two for the price of one? An Obama-Clinton ticket might give the Democrats three for the price of two.

And what about Clinton's role if the ticket won? A new administration might do well to enlist him as a special Middle East negotiator.

In the end, Obama may play it safe. He may not ask; she may not accept. But he could do worse than create a marriage of political convenience between the party's two leading figures.

Carl P. Leubsdorf is Washington bureau chief of the Dallas Morning News. Reach him at cleubsdorf@dallasnews.com.