Democrat selection of as a running mate is complicating Republican analysis of his prospective vice presidential contenders.
Biden will make his formal debut Wednesday with a prime-time address. McCain is expected to announce his pick after Obama accepts his nomination here on Thursday.
Some insiders are pressing McCain to make a strategic selection, one that beefs up his economic strength, enhances his chance to grab a state or amps up the partisan firepower.
"McCain knows Biden well. He knows how good he is as a knife fighter. He'll take McCain apart," said one Republican operative.
But a review of the much-rumored McCain shortlist clearly exposes the weaknesses each person on it might bring if matched up against the six-term senator from Delaware.
• The star of former Massachusetts Gov. Mitt Romney seemed to be rising this summer in tandem with voters' increasing anxiety about the economy.
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But a McCain gaffe over how many homes he owns - he told Politico he didn't know the exact number - would take on new life if multimillionaire Romney became his running mate.
Democrats already have calculated that the two men own a dozen homes between them, valued at a total of about $35 million.
That message could hurt McCain in two ways: It undercuts his argument that Obama is an out-of-touch elitist and would make Romney a poor match to Biden's middle-class upbringing and common-man appeal on the stump.
• Minnesota Gov. Tim Pawlenty's strength is youth and executive experience. On television talk shows, he's also shown a willingness to level attacks against Obama, although they are largely a reiteration of campaign talking points.
But Pawlenty, 48, may seem too young and inexperienced when measured against Biden, 65, who was elected to the U.S. Senate in 1972 - when the governor was just 12 years old.
During his service in the Senate, Biden has become a respected voice on foreign affairs. President Bush called him for advice after the Sept. 11 attacks.
Biden also is a two-time presidential candidate and a skilled sparring partner with years of practice against Republicans in the upper chamber - experiences that have given him a familiarity with partisan debate on the national stage that Pawlenty lacks.
A nationally televised Biden-Pawlenty debate "is unthinkable," said one Republican insider.
• Former Homeland Security Director Tom Ridge could be a better match to Biden's national security credentials. Ridge's service in the Bush administration after the Sept. 11 attacks provided a crash course on terrorism and national security issues.
As a former Pennsylvania governor, he could be a powerful counterweight to Biden's Catholic upbringing in Scranton, a working-class Keystone State enclave that went heavily for Sen. Hillary Rodham Clinton in the primaries but could be up for grabs in November.
Ridge also is capable of throwing stinging political punches and has become a mainstay on the political talk-show circuit.
But there are downsides to a Ridge selection.
He is a supporter of abortion rights, which would aggravate McCain's already uneasy alliance with his party's conservative wing.
Ridge has tried to assuage conservatives about his abortion stance by stating that he would defer to McCain's position calling for overturning the landmark Roe v. Wade ruling establishing abortion rights.
But such a nuanced position would be fertile turf for Biden to try to accomplish two goals: painting Ridge as a flip-flopper and driving a wedge between McCain and many independent and swing women voters on the abortion issue.
Another Ridge weakness is that a consulting firm he created after serving in the Bush administration recently disclosed a large lobbying contract with Albania.
Obama would seize on that contract to udermine McCain's efforts to position himself as a crusader against the professional advocacy class.
While McCain aides are pressing for a strategic pick, insiders say the independent-minded Arizona senator has approached the decision through the lens of governing: Who would add value to policy debates, and who is best prepared to step into the top spot?
• That approach tends to enhance the credentials of Connecticut Sen. Joe Lieberman, a recently turned independent who has an easy rapport with McCain and who has already run for vice president as a Democrat.
But Lieberman's long history as a Democrat could make for a bizarre debate with Biden - with the two of them sharing long records supporting labor causes and abortion rights and a host of other issues that would infuriate McCain's activist base.
In essence, said one insider, a Lieberman pick "means McCain would run a campaign without a core constituency of the Republican Party."
Phyllis Schlafly, of the conservative Eagle Forum, was more blunt: "I think there would be a walkout on Lieberman at the convention. He's not a Republican."
McCain's rethinking doesn't mean that the Biden pick doesn't open some doors.
While Biden enhances Obama's foreign policy credentials, he doesn't represent an effort to reach out to moderate voters. Both men are ranked among the Senate's most liberal members.
Brian Darling, a political analyst at the Heritage Foundation, says McCain doesn't "need to answer" the Biden pick and could take advantages of the geographic flexibility it suddenly offers him.
"If this election is going to be as close as the polls indicate, Joe Biden doesn't change the map at all," said Darling.
By Jeanne Cummings