Sen. Rob PortmanRob PortmanOvernight Finance: Senators spar over Wall Street at SEC pick's hearing | New CBO score for ObamaCare bill | Agency signs off on Trump DC hotel lease GOP senators offer bill to require spending cuts with debt-limit hikes Vulnerable Senate Dem: Border tax concerning for agriculture MORE (R) on Sunday said GOP nominee Mitt Romney could “probably” win the election without capturing Ohio, but cautioned such a path would be a huge risk to the campaign.

“He can probably win the presidency without Ohio, but I wouldn't want to take the risk,” said Portman, a prominent Romney surrogate on ABC’s “This Week.”  “No Republican has,” he added.

Ohio is one of 12 battleground states captured by Obama in 2008 where Romney will need to perform well to win the election. No GOP candidate has lost the state in a successful bid for the presidency.

Portman, who has been helping Romney prep for his debates, the second of which is slated for Tuesday, downplayed a recent poll suggesting that Obama has built a 5-point lead in the state.

“We're doing great in Ohio. If you look at the average of all the polls, it's about dead-even in Ohio right now. And importantly, the momentum's on our side. It's been terrific,” he said.

Polls after Romney’s successful first debate suggested that the race in Ohio had narrowed to a virtual tie, but a survey from Democratic-affiliated Public Policy Polling (PPP) released Saturday showed Obama with a 5-point lead, with 51 percent support to Romney’s 46.

Both Romney and running mate Rep. Paul RyanPaul RyanFox News host promoted by Trump calls on Paul Ryan to step down Dan Rather: Failure to repeal ObamaCare most 'staggering loss' so early in a term Sunday shows preview: Aftermath of failed healthcare bill MORE (R-Wis.) campaigned in the crucial battleground on Saturday.

Portman said the GOP ticket had seen a surge of momentum since Romney’s first debate with the president, one which Romney was widely seen to have won.

“I've been to a half-dozen rallies in Ohio in the last week alone. I've never seen this kind of energy and enthusiasm on the ground,” Portman said. “We've already made three times more phone calls through our volunteers this year than 2008. We've knocked on 25 times more doors than we did in all of 2008. So something's going on, on the ground in Ohio. It's turning our way.”

Ohio is the third-largest swing state prize with 18 Electoral College votes at stake.