The survey, which had an error margin of plus or minus three percentage points, found that Republicans had wiped out the Democratic edge with women, Roman Catholics, less affluent Americans and independents, key to Obama's victory in 2008 and the Democratic capture of Congress in 2006.
While Democrats have beaten Republicans among women voters since 1982, Republicans held a four-percentage-point advantage in the final campaign sprint, and a 47-32 percent edge with independents who often sway US elections.
The poll found nearly two in three respondents willing to back political newcomers on November 2, and 28 percent more ready this year to back someone with views that "seem extreme."
Obama planned an 11th-hour coast-to-coast campaign blitz through key battlegrounds in the elections, while Clinton lent his political star-power to Democrats in tight races, notably looking to pump up the party's get-out-the-vote efforts.
The former president was to attend three rallies with Democratic Representative Joe Sestak, who was locked in a bitter too-close-to-call Senate battle with Republican Pat Toomey in Pennsylvania.
All 435 House of Representatives seats, 37 of 100 Senate seats, and 37 governorships are up for grabs in the elections.
Historically, a sitting US president's party loses seats in his first mid-term elections, though such contests have not been good predictors of chances for a second term.
Taking control of even one chamber of Congress would give Republicans broad control over the legislative agenda in Washington and new power to investigate the Obama administration and probe government programs.
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