The coalition that powered Obama's historic candidacy four years ago is still broadly supportive of the president, but concerns over job prospects and the stagnant economy have seen enthusiasm wane from 2008's dizzy highs.
"He has lost support to some degree across the board with the exception of Hispanics and African Americans," Scott Keeter, director of survey research at the Pew Research Center, told AFP.
"The question of whether the different parts of his 2008 coalition turn out at the same rates they did four years ago is vital and one that nobody knows the answer to at this point."
More than 95% of the record African American turnout in 2008 voted for Obama. He also won the fast-growing Hispanic vote by a margin of 67-31% and Asian Americans by 62-35%.
Two-thirds of those under 30 voted for Obama and provided the energy behind his Internet-savvy campaign. He also captured 56% of women, compared to the 51% won by Democrat John Kerry in 2004.
Core support for Republican presidential candidates comes from white voters concerned about taxes and wary of excessive government spending, as well as from religious conservatives who oppose gay marriage and abortion.
Republican challenger Mitt Romney, a multimillionaire former venture capitalist, has broader appeal despite a brutal primary battle that forced him to swing to the right on immigration and social issues like abortion.
"He's not depending solely on an ideological base or a conservative Christian base at all to get him to the point where he is neck-and-neck with the president," said Keeter.
Romney holds a slight lead over Obama in national polls, but the president has a slender advantage in the swing states that will actually decide the November 6 election.
With the key states of Florida and Virginia highly vulnerable, Obama's "firewall" could end up being Midwestern states that are overwhelmingly white such as Ohio (83.6%), Iowa (93%) and Wisconsin (88.4%).
Ohio is the big prize as it has the most electoral college votes on offer and both candidates are engaged in a ferocious battle for the "Buckeye State".
Obama won Ohio by a margin of 51.5% to 46.9% over John McCain in 2008, thanks largely to strong support from the usual suspects: young women, African Americans and other non-white minorities.
"This time there's clearly in Ohio a decline in enthusiasm for the president among these groups," political expert John Green, from the University of Akron, told AFP.
Obama's successful bailout of the automobile industry and Romney's opposition to that bailout are crucial factors in Ohio, which has the second-highest number of auto-related workers in the country after Michigan.
With his coalition fraying, Obama has bombarded the airwaves in Ohio all summer long with ads portraying Romney as a ruthless corporate raider who would sell off US jobs overseas and doesn't care about ordinary Americans.
Despite the fact that Obama lost working-class whites by 18 points in 2008, he appears to be courting them in Ohio as a last line of defence.
"I don't know if that's the actual strategy of the campaign but the evidence certainly fits that pattern," said Green.
Polls suggest Obama's anti-Romney ads found fertile ground in Ohio, where unemployment has fallen below the national average and the economic environment is much improved.
"Obama has something to work with those white working-class Ohioans," Green said. "A group of voters that were pretty turned off to the president a year ago now feel better about him."
An ABC/Washington Post survey out on Friday showed that Obama is polling seven to 12 points better among white men in Ohio than he does nationally.
Keeter said it was ironic that with his 2008 coalition flagging, Obama may now be reliant on working-class white votes in states like Ohio to secure a second term.
"I can't deny there is something quite interesting about the fact that he is staking so much on the Rust Belt states and that it makes sense," he said.
The Democrats' greatest fear is that the enthusiasm gap means their core support doesn't turn out on election day.
Hope and change mantra
Obama's 2008 victory came against the backdrop of deep antipathy towards outgoing president George W. Bush, a lackluster McCain campaign, and huge excitement at the possibility of electing the nation's first black leader.
His hope and change mantra is now just a T-shirt and many Americans are struggling to find jobs or pay their bills, deeply skeptical of the nascent recovery.
But even if the Republicans and Romney profit from the economic malaise and pull off an unexpected come-from-behind win on November 6, experts say they should take heed of the longer-term demographic trends.
The Republican base of white voters is contracting, college-educated whites are turning increasingly Democratic, and the percentage of Americans living in socially conservative rural areas is also shrinking.
The white share of the electorate – 90% in 1976 – dropped to 74% in 2008 and is due to plummet to 46% by 2050, when minorities become a majority and nearly a third of the population will be Hispanic. – Sapa-AFP