"Political scientists have looked long and hard for evidence of a vice presidential effect on the electorate, and they haven't come up with much," said William A. Galston, a senior research fellow at the Brookings Institution, in an article online.

Galston gave as an example the 1988 presidential election, when Democratic presidential nominee Michael Dukakis selected highly qualified Texas Senator Lloyd Bentsen as his running mate, while George H. W. Bush chose a lightweight Indiana senator named Dan Quayle. Bentsen trounced Quayle in their only debate, but Bush led in the polls by late summer and later won the election.

According to Gallup Poll Daily, the vice presidential selection did not give Obama a bounce in the polls. Tracking from August 23 to 25, the first three-day period following the announcement, showed 46 percent of registered voters nationwide supporting McCain and 44 percent supporting Obama, a marginal change from the previous week.

Voters had a similar reaction to Alaska Governor Sarah Palin's selection as the Republican vice presidential nominee on August 29-"mildly positive but also with a high degree of uncertainty about each," Gallup pointed out.

After all, Biden's name is unfamiliar to most Americans, even though he ran for the Democratic presidential nomination in 1988 and again this year. He withdrew from the race in 1988 amid accusations that he plagiarized an article he wrote as a law student. Biden ended his 2008 primary campaign in early January after winning only 1 percent in the Iowa caucus.

These two national campaigns make Biden by and large a safe choice for Obama, since the intense press scrutiny he has received in the past "lessens the chances of some sort of revelation over the next few months that takes Obama off message," said Chris Cillizza, who runs a politics blog on The Washington Post website. The McCain campaign, by contrast, has had to contend with a series of embarrassing revelations about Palin, a largely untested candidate.

Biden also brings far more than his foreign policy experience to the Democratic ticket. Born into a Catholic family in Scranton, Pennsylvania, a working-class town in a crucial swing state, Biden appeals to voters who view Obama with uncertainty.

Pundits say Biden could help Obama lock down Pennsylvania, Michigan and other blue-collar states. "Biden is someone that will play well with older white voters, a demographic group Obama's struggled with, and he's very popular with labor and trial lawyers," NBC political analysts wrote on August 23.

Outspoken and charismatic though he is, Biden has an unfortunate tendency to put his foot in his mouth. He caused an uproar in 2007 when he described Obama as "the first mainstream African-American [presidential candidate] who is articulate and bright and clean and a nice-looking guy." In 2006 he offended Indian-Americans with his comments about population changes in Delaware, "You cannot go to a 7-Eleven or a Dunkin' Donuts unless you have a slight Indian accent."

The McCain campaign has already rolled out ads using footage from the Democratic primary campaign in which Biden questions Obama's readiness to be president. "Senator Obama has chosen Senator Joe Biden as his running mate, a man who doesn't believe he's ready to lead as president. Senator Obama's poor judgment calls indicate he's not ready to lead as president," said Rick Davis, McCain's Campaign Manager, in an e-mail to voters shortly after Obama announced his vice presidential pick.

Another question is whether Biden's presence on the Democratic ticket is consistent with Obama's message of change. Obama has been campaigning against Washington insiders, which would presumably include a long-serving senator like Biden.

Are Obama and Biden a good match? It seems so far so good. The two began their first campaign swing together immediately after the close of the convention, charming audiences in Pennsylvania, Ohio and Michigan. A September 1 report in The Washington Post quoted a senior Obama aide saying that even Obama has been surprised by their chemistry on the campaign trail.

(Reporting from New York)