Barack Obama has a problem, and it's not just that he trails far behind Hillary Rodham Clinton in the race for the Democratic presidential nomination as the campaign heads into its final months.

Of course, nothing is certain in politics. Sen. Obama of Illinois could turn things around. But he trails by more than 20 points in national and most state polls. Not only that, Sen. Clinton's supporters, according to polls, are more committed to her than are those who support other Democratic candidates.

Given the increasing odds against an Obama comeback, the senator is certainly aware that he needs to consider his political future while managing his political present.

In other words, unlike virtually the rest of the Democratic candidates, whose futures in presidential politics are very limited, Obama could lose the nomination and, despite the lesson of party history, live to fight another day.

Democrats generally banish their unsuccessful presidential candidates. The last nonincumbent to win the party's presidential nomination on a second try was Adlai Stevenson in 1956, and he won it the first time he tried in 1952.

Republicans are different. You might remember some guys named Richard Nixon, Ronald Reagan and George H.W. Bush, all of whom won the GOP nomination and the presidency on the second, or in Reagan's case, third try.

The idea that Obama could lose the Democratic nomination to Clinton in 2008 and come back to win it in either 2012 or 2016 stems from his age — 46 — and the fact that he is new to the national political scene.

Obama was elected to the Senate in 2004, and his fresh face image is both an asset and a liability in a presidential campaign. Polls show voters think he may not have enough experience for the job yet see him as an agent of change, which they like.

None of the other crop of Democratic hopefuls, with the possible exception of New Mexico Gov. Bill Richardson, would seem to have any hope of winning a subsequent nomination should they lose this time. For the most part, these other candidates would have little to lose if they were to alienate Clinton en route to her nomination.

But because Obama could have a second act on the presidential stage someday, he needs to be very careful in how far he goes in alienating Democrats who might support him in the future with his campaign tactics this time.

That's a lesson learned from John McCain, the Arizona Republican who, it turns out, alienated a lot of conservatives in his 2000 run for the nomination against George W. Bush.

McCain is paying for it this year as he seeks to be the GOP designee.

McCain entered the year as the GOP front-runner, but his inability to connect with party conservatives, whom he scorned in 2000, has cost him and left him a long shot for the nomination.

The paradox for Obama is that it seems unlikely that he can defeat Clinton unless he runs a strongly negative campaign against her. But if he did so and lost anyway, then he might find his image for future runs tarnished among Democratic activists. And if he were to wage a strongly negative campaign, it might change the general public perception that he is not a typical politician, which is a political asset for him.

There is also the question of the vice presidency. Personal chemistry between the presidential candidate and his or her running mate is not necessarily a requirement. Nevertheless, a nominee rarely picks a vice president who has been harshly critical of the victor during the primaries, or with whom there are bad feelings.

Leaving aside the wisdom of whether he would be the best choice for Clinton should she win the nomination, political common sense argues that Obama would be foolish not to keep that option open.

Thus, Obama needs to avoid alienating Clinton and her supporters through attacks they might consider beyond the pale at the same time he tries to convince many of those same folks that she isn't as wonderful as they think.

Peter A. Brown is the assistant director of the Quinnipiac University Polling Institute. Reach him at peter.brown@quinnipiac.edu. This article first appeared on www.Politico.com.