Despite a strong debate performance, Romney's path to the presidency remains narrower than Obama.
Particularly worrisome for Republicans is Ohio, a state that every Republican presidential candidate has needed to win the White House.
If Obama wins Ohio's 18 electoral votes, Romney would need to win Florida and in all likelihood secure several up-for-grabs states such as Virginia, Iowa, Colorado, New Hampshire and Nevada to take the White House.
Romney, following his selection of Wisconsin Rep. Ryan as his running mate, has tried to put Wisconsin into the toss-up category, but public polling has shown Obama ahead, giving the president more breathing room.
In the season of debates, next up is the only match-up between Vice President Joe Biden and Ryan.
A strong performance by Biden, a former senator who essentially made a career out of debating colleagues, could quell nervousness among some Democrats, though neither party expects undecided voters to be swayed by the face-off between the running mates. Ryan's challenge is to overcome his lack foreign policy expertise or national debate experience against Biden, who has extensive experience on both fronts.
"Believe you me, I understand this man is extremely experienced, he's a gifted speaker, he's a proven debater," Ryan said on "The Frank Beckmann Show" on Detroit radio station WJR. "So we definitely have our work cut out for us. But the problem the vice president has that he just can't get around is he has to try and defend Barack Obama's record, and it's not a very good record to defend."
Biden was preparing for the face-off in his hometown of Wilmington, Del., where he has held two mock debates with Rep. Chris Van Hollen, D-Md., who is playing the role of Ryan.
Obama and Romney will face off again on Oct. 16 in Hempstead, N.Y. in a town hall debate.
Associated Press writers Stephen Ohlemacher, Steve Peoples, Ben Feller, Ken Thomas, Nedra Pickler and Jim Kuhnhenn contributed to this report.