On the one hand, Biden, D-Del., is an uncontroversial and indeed impressive choice who perfectly complements Obama, D-Ill., much as Republican Texas Gov. George W. Bush's selection of former Defense Secretary Dick Cheney from Wyoming reassured centrist voters in the 2000 presidential race.

Like Cheney eight years ago, Biden comes with a long resume of more than a quarter-century on the national scene and with credentials in foreign policy that are seen as balancing the domestic political savvy but lack of national experience of the prime candidate.

However, the pick of Biden also carries a very significant downside for Obama.

First, Obama, the "candidate of change," already has been hemorrhaging idealistic liberal voters and disgruntled Hillary Clinton supporters at an alarming rate. Obama's refusal to select not just Clinton, D-N.Y., but any other woman as his running mate will exacerbate that trend.

Recent polls have indicated that as many as 20 percent or more of the 18 million registered Democrats who voted for Clinton during the long, grueling primary race have decided to vote for McCain. If those figures hold and Obama cannot win them back, he will lose in November. Biden gives him no pull at all with that group.

And there were at least two first-class potential women running mates Obama could have chosen: Kansas Gov. Kathleen Sebelius, who is respected throughout the Midwest, and Arizona Gov. Janet Napolitano, who could have forced the Republican presidential standard-bearer, Sen. John McCain of Arizona, to defend his home turf. By passing them over too, Obama gave another slap in the face to feminists already wary of him.

Second, Biden is supposed to provide reassurance to centrist voters, but his voting record in the Senate has always been reliably very liberal. Third, pro-Israel supporters regard him as insufficiently supportive of their cause.

Older, pro-Israel voters, including many registered Democrats, tend to congregate in crucial Florida, the fourth most populous state in the nation. Al Gore lost Florida by a hairsbreadth -- and with it the presidency -- in the disputed, cliffhanger 2000 election. With the wealthy, politically active and influential Cuban community in Florida already firmly in the GOP camp, picking Biden only compounds Obama's Jewish voter problem in Florida. That, in turn, frees McCain and the Republicans to use lots of their resources elsewhere.

Fourth, Biden is usually explained on the grounds that he can appeal to white, working-class male voters. But there has never been any evidence whatsoever that the intellectual, serious Biden has ever had any pull with working-class voters outside his own state of Delaware.

Biden's runs for the presidency over the years have never gotten off the ground in terms of popular support. He cannot even exert any significant influence in neighboring Maryland, Pennsylvania or New Jersey -- all significant Electoral College prizes in November. By contrast, Sebelius had pull throughout the Midwest, and Napolitano on the ticket could have helped Obama in every border and Southwest state.

Fifth, Obama's belief that Biden can serve as his attack dog against McCain suggests a desperate grasping at straws and may very well boomerang back on the Democratic nominee.

Obama owed his early appeal to his pledge to run a clean campaign with inspiring rhetoric that accentuated the positive and eliminated the negative, as the great songwriter Johnny Mercer memorably wrote. However, getting Biden to go negative against McCain will make a mockery of that strategy. Indeed, it may backfire, by making Obama look reluctant or fearful to engage McCain more directly himself.

Sixth, Biden shares one of Obama's most defining characteristics, which is already backfiring on the senator from Illinois: Both men are intellectuals and lawyers by training who love to give lengthy speeches. Obama's set-piece rhetoric gave him that crucial early boost in Iowa and afterward allowed him to seize the nomination from Clinton, the longtime front-runner. But his failure to back up his inspirational rhetoric with a detailed command of issues, especially on national security, energy and plausibly balancing the budget and restoring national fiscal stability have cost him badly in recent weeks. Pollster John Zogby has tracked a 12-point reversal in support at Obama's expense in the month of July alone.

Far from helping Obama with substance, Biden's old-fashioned, orotund rhetorical style may only make both men look old and out of date compared with McCain's feisty wit.

Nor will Biden's choice reassure centrist voters about Obama's moderation. On many, if not most, issues, Biden has a very long record of voting far more to the left than Obama. This will cripple Democratic efforts to reassure voters that Obama is a mainstream moderate and not a closet leftist. The Republicans are bound to take maximum advantage of that.

Finally, it was ironic that Obama used his gimmick of texting supporters with the red-hot news of his vice presidential choice, only to reveal that it was going to be one of the most familiar and venerable figures over the past three decades in the United States Senate, and one who, for all his experience, has never set the world on fire before.

A Gallup poll conducted Saturday found that Obama's choice of Biden gave the Democratic presidential contender a mere 7-point potential edge among voters. In 2004 Sen. John Kerry, D-Mass., got a 17-point boost when he chose Sen. John Edwards, D-N.C., as his running mate.

Obama still faces the challenge of getting back his momentum -- his "big mo" -- at the Democratic National Convention in Denver this week. So far his vice presidential pick hasn't helped him.