Should Romney fall short, the list of Republicans who could step up is lengthy.
Ryan, this year's vice presidential nominee, would be toward the front of the pack if he ran. So, presumably, would a slate of governors — Chris Christie of New Jersey, Nikki Haley of South Carolina, Bobby Jindal of Louisiana, John Kasich of Ohio and Scott Walker of Wisconsin. From the Senate, Kentucky Sen. Rand Paul could play for the tea party mantle, with the latter viewed as the heir to father Ron Paul's loyal base of libertarian-minded followers. So could Rubio, a GOP sensation only two years into a Senate term who is headed for Altoona, Iowa, later this month as the featured guest at a birthday celebration for Republican Gov. Terry Branstad.
Former Pennsylvania Rick Santorum, the runner-up to Romney, could give it another go. And former Florida Gov. Jeb Bush hasn't ruled anything out.
Virginia alone could see three former governors as contenders: current Democratic Sen. Mark Warner, former Democratic National Committee Chairman Tim Kaine if he prevails in his Senate race next week and Republican Gov. Bob McDonnell, who is limited by law to one term that expires after next year.
Assuming Romney wins, Iowa-based Republican strategist Eric Woolson said he expects the initial forays by Democrats to begin within a few months of the inaugural.
"It will be next spring that candidates will start to come in and we'll start to see the 'is he/she or isn't he/she running for president? They must be, otherwise they wouldn't be in Iowa' stories," Woolson said.
Iowan Jackie Norris, a former Obama campaign adviser, sees it as "a wide open year ripe with opportunities for veteran and newcomers alike. After two successive terms it's the party's opportunity to create the post-Obama view of the future. "
Part of the exercise for would-be candidates is figuring out how to slip into a presidential proving ground, make a solid impression and mask any White House ambitions by telling anyone who asks that a campaign is a distant proposition. Part of the calculation is about how to afford it all in a campaign finance system stripped of many regulations by the courts.
Part of the challenge: introducing themselves to voters likely not ready to turn to the next campaign.
Bakst reported from Richmond, Va. Associated Press writer Matthew Daly in Washington contributed to this report.