Demographics part of calculation

Then again, neither Romney, who has backed abortion rights in the past, nor Ridge, who supports them now, has been embraced by the GOP's social conservative base.

Democrats seemed to rally around Biden as a strategically smart choice.

While the two men seem a study in contrasts -- young vs. old, black vs. white, political newcomer vs. grizzled veteran, Harvard Crimson vs. Fightin' Blue Hens of the University of Delaware -- Obama aides hope the Biden biography can help sell skeptical voters on the lesser-known parts of Obama's story.

Obama may be known for lamenting the high price of arugula at Whole Foods, but Biden took pains Saturday to talk about his difficult upbringing by a "single mom who had to struggle to support her son and her kids."

Obama aides said they were attracted to the Delaware senator's humble beginnings -- and his relatively modest lifestyle. He has a net worth between $59,000 and $366,000, not much for the millionaire's club known as the Senate.

"He's still one of the poorest members of the Senate," said Anita Dunn, a senior advisor to Obama's campaign. "He came to Washington to do good, not to do well for himself."

Biden arrives on the ticket at a time when some of Obama's political vulnerabilities seem to be taking a toll. A once-healthy lead in most polls has dwindled to a statistical tie after weeks of attack ads by McCain and the GOP portraying Obama as a celebrity out of touch with the needs of everyday people.

The weakest links in the Democratic coalition are whites, Catholics and blue-collar voters -- what would seem to be a tailor-made audience for Biden.

About 10% of respondents to a Wall Street Journal/NBC poll said they would have voted for Clinton against McCain but will not back Obama. That group skews female, Catholic, Democratic and/or lower-income, according to GOP pollster Neil Newhouse, who conducted the survey with Democratic pollster Peter Hart.

Although Clinton supporters remain raw over the competitive primary campaign and had still hoped the New York senator would be named to the ticket, Democratic strategists said Biden was a safer choice than others under consideration.

"Most of those who are disappointed that Hillary Clinton was chosen will readily concede that Joe Biden is a solid choice and highly qualified -- avoiding some of the hard feelings that might have occurred if some of the lesser experienced contenders had been selected," said Geoff Garin, who was a top strategist for Clinton's campaign.