What a difference a debate makes.

Republican Mitt Romney has opened a large, 7 percentage-point lead over President Barack Obama in must-win Florida, according to a new poll of likely voters conducted for The Miami Herald/El Nuevo Herald and the Tampa Bay Times.

Romney’s 51-44 percent advantage is just on the cusp of the poll’s error margin — and it marks a dramatic 8-point shift since last month.

“Obama’s now swimming upstream,” said Brad Coker, pollster with Mason Dixon Polling & Research, which conducted the survey of 800 likely Florida voters this month and last for The Herald and its news partners, including Bay News 9 and Central Florida News 13.

The previous poll, which showed Obama with an inside-the-error-margin lead, was before last Wednesday’s debate when Obama gave a lackluster performance while Romney appeared to excel.

This latest poll showed that 5 percent of those who said they were undecided before the debate say they’ll vote for Romney. And 4 percent of those who said they favored Obama pre-debate moved away from the president — 2 percent toward Romney and 2 percent undecided.

“Obama didn’t flip one voter,” Coker said. “He didn’t gain 1 percent from the debate.”

Even Democrats were upset with Obama’s performance.

“I was disappointed,” said Phyllis Apple, a 90-year-old Democrat from Aventura. “He didn’t look like he was ready to fight. Maybe the president thought it wouldn’t look presidential.”

A top political advisor to the president, David Plouffe, acknowledged that the debate was a “wake-up call” and that the race has tightened. But, he said, he believes in the campaign’s message and its vast volunteer army that can turn out nontraditional voters who don’t necessarily get picked up in polls such as this one.

“That’s where there are real gains for us. We’ve got to find them,” Plouffe said. As for Romney’s surge, Plouffe said, “Romney picked up his easy gains ... We think the Romney gains have stopped.”

Romney’s strong performance also led people to trust him more on Medicare, the economy and foreign policy, which once was a strong suit of the president after the killing of Osama bin Laden.

But the fallout from the Sept. 11 terrorist attacks in Libya — not just the debate — might have hurt Obama as well. Also potentially helping Romney is that the economy in Florida is stuck in the doldrums, with the unemployment rate ticking up slightly last month while it fell nationally.

Romney’s biggest gain: independent voters, the swing electorate in swing-state Florida, without which Romney has no shot of winning the presidency this year.

Independents moved 24 points away from Obama and toward Romney, who’s now favored by 13 points, or 52-39, over the president.

Another huge gain came with women, who once favored Obama by double digits, and are now virtually split between the top two contenders. They moved a total of 13 points in Romney’s favor in a month.

In a sign of his need to reach out to women, Obama’s campaign has stepped up its criticisms of Romney for his opposition to abortion. The Democrats have sent out stacks of mailers to registered Florida female voters reminding them of Romney’s stances and have suggested that the Republican would want to ban some forms of birth control.

Obama is now winning just one of the state’s five regions, liberal Southeast Florida. But he once led by 21 points here — now it’s down to 12 percentage points.

Generally speaking, Democrats win the South and Republicans win the North. That makes Tampa Bay and Central Florida the swing state’s swing areas. Romney is winning the former now, 52-44 percent, and he’s up in the Orlando area, 49-39.

Old and young voters didn’t move very much. But middle-aged voters — the bulk of the Florida electorate — shifted almost 12 points in Romney’s favor.

Romney even started to gain more support from Hispanic voters, who moved 11 points in Romney’s favor.

The majority of the electorate, non-Hispanic white voters, were always in Romney’s corner. Now they’re even more solidly behind the Republican, backing him 61-34 percent over Obama.

“All of this is tied to that debate,” Coker said. “There’s no question in my mind that that debate really made people stand up and pay attention and it really wiped away questions people might have had about Romney who were either undecided or soft for Obama.”

The election is far from over.

Though Romney made up major ground, he and the president have two more debates and four weeks of campaigning to go. Also, Vice President Joe Biden and Romney’s running mate, Congressman Paul Ryan, debate Thursday night. The effects of that face off won’t be known for a few days hence.

Still, Coker said all the trends portend a steep climb for Obama because it shows that Romney is appealing to the center of the state.

Romney is trusted more than Obama on looking out for the middle class: 50-47. On the economy, 50-44 percent. On foreign policy: 49-46 percent. On leading the nation: 51-46 percent.

While Democrats expressed their disappointment, Republicans like Ellen Larmoyeux were thrilled with the last debate.

"My husband and I are for Romney, and after seeing the debate last week, it solidified any doubts we might have had,” said Larmoyeux, 55 and of Jacksonville. “Once we saw the two of them next to each other, the choice was even more clear."

The poll showed that Romney picked up some Democratic support and that Obama lost it. Romney gained a few extra Republicans as well; Obama didn’t.

Another poll released Thursday, from NBC and the Wall Street Journal, showed different results from The Herald’s survey conducted by Mason-Dixon. That poll showed Obama with a 1-point, inside-the-error margin lead over Romney.

One difference between Mason-Dixon — a nonpartisan Jacksonville-based firm — and other polling firms is that Mason-Dixon specifically identifies how respondents are registered to vote. It’s an important question in a state where party registration is clearly defined, which it isn’t in other states.

Democrats, who outnumber registered Republicans in Florida, comprised 44 percent of this poll’s sample. Republicans comprised about 40 percent. Independent voters accounted for 16 percent.

This poll also might not pick up many of the more than 300,000 new voters registered by the Obama campaign this year. The campaign is aggressively trying to bank early votes by encouraging more absentee-ballot voting.

On Wednesday, the Obama campaign encouraged supporters to request absentee ballots, fill them out and drop them off in person at local election offices.

Independent voters like Matthew Marino had their minds made up well before the debate. They’re siding with Romney.

“I will be voting for Mitt Romney because I do not believe President Obama’s vision for the country will grow the economy and reduce the debt,” Marino said. “More taxes on those who are working and more government control are not the answer.”

Coker compared the debate to the face-off between Ronald Reagan and President Jimmy Carter in 1980, when the incumbent lost in a bad economy.

“The only problem for Romney is this debate didn’t occur five days before the election like the Reagan-Carter one did,” Coker said. “He still has to navigate two more and there’s another four weeks of campaigning. So there may not be lasting effect. But there has been an immediate impact.”