After cinching the nomination and getting Hillary Clinton’s endorsement, everyone expected Barack Obama to have a substantial bounce in the polls. Not only did the uptick seem paltry at seven, it has shrunk to less than half of that number within the week. The daily Gallup tracking poll now shows John McCain within the margin of error, only three points back:

Barack Obama leads John McCain in national registered voter preferences for the election, but by a slightly narrower margin than he had earlier this week, 46% vs. 43%.

Although Obama’s three percentage point advantage is statistically significant, it is down from the 6- to 7-point leads he had in Gallup Poll Daily tracking reports on Monday, Tuesday and Wednesday of this week. (To view the complete trend since March 7, 2008, click here.)

What’s remarkable about this poll is how closely McCain has tracked with Obama over the last several weeks. Despite mostly being out of the media limelight, he has only trailed by seven at the height of the Obama bounce. For most of the last three weeks, the gap has been within the margin, and McCain has even led slightly at times. The race appears ready to return to that status much more quickly than anyone would have guessed.

That doesn’t bode well for Obama, neither in the length nor the amplitude of his bounce. It suggests that even all of the laudatory coverage of his historic nomination, as well as his remarkable achievement in beating the Clinton machine, cannot mask the weakness of his candidacy. His series of stumbles that started in April continue, and with only him and McCain in the race, they will get magnified. Meanwhile, McCain will start gaining more attention from the media, having survived the drought in fine fashion.

Team McCain has to be pleased with his relative position. Obama’s campaign should have more concerns about his staying power.