According to the latest Gallup Poll, Barack Obama's selection of Joe Biden as his pick for vice president hasn't yielded the bounce most candidates enjoy after announcing their running mates. In fact, the poll shows the opposite_ McCain has erased Obama's lead and is now ahead, 46 percent to 44 percent.

Psychologically, the drama of a candidate for president teaming up with another leader to do battle in the last months of the election should provide a burst of enthusiasm among voters, however short-lived. The Democratic National Convention, playing like rock music in the background, should add plenty of fuel to carry more people into the next chapter of the Obama-Biden story.

Engagements and weddings and political conventions are times for unbridled optimism. The audience, which includes the American public in this case, is predisposed to believe that human beings joining together can be much more than any single person could ever be alone, that the glistening start of a partnership predicts sure success.

But something is wrong, and I think I know what it is. The Barack Obama story itself is the stuff of big, big dreams. We've watched a first-term U.S. Senator capture the imagination of the nation with eloquence unparalleled in recent times, harking back to the kind of excitement John F. Kennedy generated. We've watched him defy the odds again and again, to stand at the zenith of the Democratic Party. He has stood, in fact, somehow above and beyond traditional politics, a larger than life figure, a phenomenon. Those who embrace him hope for-maybe even expect-miracles from him.

So it should come as no surprise that selecting a respected, tested running mate like Joe Biden would slow Obama's momentum a bit. While the choice may reassure voters that a steady hand with vast experience will be helping chart a course through increasingly stormy economic and political seas, it also reminds us that Obama is himself a politician facing momentous challenges. Joining hands with a longtime U.S. Senator anchors Obama, and voters, to reality. It brings Obama back down to earth. It makes him seem human, where he once seemed superhuman.

Only one pick would have taken Obama further beyond the normal gravity of the political universe: Hillary Clinton. In inviting his former rival onto the ticket, in trying to help bring the first female vice president into the White House, he would have been reaffirming the notion that he cannot be defined or limited by tradition or expectation. He would have been saying that he could achieve anything.

Hillary Clinton joining hands with Obama was the chapter that many Americans had already written into their collective imaginations, and those imaginary pages had much more energy than the real ones we're reading about Obama and Biden.

As we wind our way toward November, Obama has come face-to-face with this reality: America's expectations of a phenomenon are quite different than its expectations of a candidate. They are limitless. And they require constant feeding.

Dr. Keith Ablow is a psychiatry correspondent for FOX News Channel and a New York Times bestselling author. His newest book, "Living the Truth: Transform Your Life through the Power of Insight and Honesty" has launched a new self-help movement. Check out Dr. Ablow's website at livingthetruth.com.

Dr. Keith Ablow is a psychiatrist and member of the Fox News Medical A-Team.