The Obama campaign suffered a major blow only hours before the vice-presidential debate when a new poll showed Romney opening up a seven-point lead over Barack Obama in Florida, the biggest of the swing states.
It is one of the most devastating set of poll figures the president has yet faced.
Were he to take Florida, with its 29 electoral college votes, Obama would be almost guaranteed re-election to the White House, needing just one more of the swing states. But without Florida, the race is thrown wide open.
The Mason-Dixon poll for the Tampa Bay Times/Bay News 9/Miami Herald showed a dramatic change from a month ago when Obama was ahead of Romney 48% to 47%. The new poll has Romney with 51% of likely voters and Obama only 44%, with 4% undecided.
Brad Coker, who conducted the poll, attributed the swing to Obama's lacklustre debate performance, with 5% of previously undecided voters moving to Romney as a result and 2% of Obama supporters saying they were now undecided.
If Florida goes to Romney, the chances are that North Carolina, Virginia and New Hampshire – four of the eight swing states – could go, too. The final result could come down to Ohio.
Almost as bad for Obama as the headline figures is the detail. Earlier polls had shown voters over 65 worried about Republican vice-presidential candidate Paul Ryan's plans for Medicare but the new poll showed 54% viewed Obama as a bigger threat to Medicare than Ryan.
Obama's lead among women, one of his strengths, has been cut to 2%, from 15% a month ago, and Romney has also achieved positive favourables among Latinos, another demographic group that has been strongly behind Obama.
The poll adds to pressure on vice-president Joe Biden to stop the slide with a strong performance in tonight's debate with Paul Ryan.
The Obama campaign team is looking for a clear win by Biden in the vice-presidential debate. A win for Ryan would maintain the Republican momentum started by Romney last week.
Biden spent the afternoon with family and friends at the house of supporters outside Danville.
The vice-president is notoriously verbose and also gaffe-prone, but he has proved to be a canny debater in the past, with a likeable, folksy style.
Ryan, in an interview with CNN to be broadcast just before the debate, attempted to lower expectations. "This is new for me. Joe has done this for decades; [he has] run for president, done all of those presidential debates, and he is sitting vice president. So, he clearly goes into this with a whole lot more experience than I do, and this is the first time I'm on a stage like this," Ryan said.
For tonight's debate, Biden has been preparing on a detailed mock-up of the set. It was recreated in a Sheraton hotel in Wilmington, Delaware, his home state.
The 90-minute debate is divided into 10-minute segments dealing with a mixture of domestic and foreign affairs issues.
In an attempt to boost Democratic spirits before the debate, the Obama campaign manager Jim Messina released details of Democratic success in the battle with Republicans to register voters and to get out early voters in the swing states.
Messina, in his briefing with reporters, talked up the Democratic successes in the swing states. "We gauge our ground game on verifiable numbers that tell the real story: voter registration and early voting," Messina told reporters. "In nearly every battleground state, margins on both counts are bigger than they were in 2008. We've registered more voters than '08, we've knocked on more doors and we've talked to more people."
Although the figures are independently confirmed by non-partisan state organisations, they can be misleading. Republicans traditionally do not go in for early voting but can be guaranteed to turn out in strength on polling days.
Democrats, by contrast, can be more difficult to motivate on polling day itself.
The Obama campaign said that registration was up among Latinos and the young. It claimed the Romney campaign was coming to the ground game late.
According to the Messina statement, Democratic party registration leads Republicans in almost every battleground state, Latino registration is higher than among whites, and most new registered voters are younger than 30.
In Iowa, the Democrats lead in vote-by-mail ballots cast, in-person early voting, total voting and total ballots requested. We also lead by a wider margin than we did at this point in 2008 in both ballots requested and
In Ohio, the Democrats lead in ballots requested.
guardian.co.uk © Guardian News and Media Limited 2010
image: © BeckyF