The same poll had Obama in a statistical tie with McCain in Ohio, leading 43-42, with Clinton enjoying a more comfortable 48-39 lead in a hypothetical matchup with McCain in that state.
Quinnipiac also has Clinton running stronger than Obama in Pennsylvania -- a state carried by Al Gore in 2000 and John Kerry in 2004, and that Democrats consider crucial to their prospects this year.
Obama supporters counter that the Clinton campaign is wrong to focus on primary results as evidence of candidates' potential strength in a general election.
While Democrats are choosing between two strong Democratic candidates in primaries and caucuses, the choice will be between McCain and only one remaining Democrat in the fall.
The Obama campaign is confident that it will ultimately be able to capture voters who are now supporting Clinton; one supporter likened it his position to starting with the support Kerry received in the 2004 campaign.
And Obama wants to build on those numbers by bringing some Republicans and large numbers of independents to his side. That, he argues, could pose a tougher challenge to McCain, since he enjoys wide appeal among independent voters.
"Obama is saying, 'The proof is the pudding,' and he's leading this race," said Celinda Lake, a Democratic pollster and strategist. "They see they can open up more of the electorate -- and certainly out West, where he is far less polarizing. What she says, meanwhile, is 'I have the experience, and experience in national security.' "
The Obama campaign has also argued that only a fresh voice -- without the mixed political legacy of the Clinton years -- can take on a candidate who himself represents a break with the status quo, in McCain.
A New York Times/CBS poll released Friday found that 57 percent of Democrats think Obama stands the better chance of defeating McCain, compared with 28 percent for Clinton.
But it's far too early to use polls to make arguments about a general election that's still seven months away, said Mark Mellman, a Democratic pollster who is unaligned in the race.
"The truth is unless you've been endowed with the gift of prophecy, it's a fool's errand," Mellman said. "We just don't know who's more electable, and anyone who does say they know is making it up -- probably out of self-interest."
Given the current state of the race, the electability argument is particularly critical for Clinton to make.
She has fallen well behind in both the popular vote and the delegate race, and her only realistic path to the nomination involves convincing superdelegates to brush aside those numbers and flock to her side.
"The whole point is for delegates, however they are chosen, to really ask themselves who would be the best president and who would be our best nominee against Sen. McCain," Clinton said Thursday.
Electability was a big source of support for Kerry in 2004, and could be equally potent this year, said Erik Smith, a Democratic political consultant.
McCain has many Democrats nervous -- and has Democratic voters and superdelegates thinking critically about matchups.
"The more assured Democrats feel they can win, the more they'll go with their hearts rather than their heads," Smith said. "McCain can separate himself from Bush on everything except Iraq. Electability becomes more of a pressing concern, a higher-priority concern among Democratic voters."