"You've got a Republican/independent Romney base, in my view, that politically speaking and policy speaking fears Barack Obama and where he's taken the country," Mueller said. "So if you have this come down to the court system, I think it'll be even messier than Bush and Gore, who weren't really seen as far lefties and far righties, whereas our side sees Barack Obama as an extreme left-wing socialist."

In a possible foreshadowing of disputes over a tight vote count, prominent voices on the right recently challenged the authenticity of September unemployment data released by the Bureau of Labor Statistics, an organization that had not previously come under partisan accusations.

And even if it is widely regarded as legitimate, an especially narrow victory by either side might further hinder whoever becomes president from gaining bipartisan cooperation over the next four years.

Asked how he and fellow prominent conservatives would regard an Obama victory that came without a popular-vote majority, Citizens United President David Bossie did not mince words.

"President Obama would be a lame-duck president on Day One of his second term," he said. "He would not have any mandate to initiate policy, and power would reside in Congress rather than the White House."

On the left, widespread allegations of voter suppression likely would emerge if Romney loses the popular vote but wins the presidency.

Citing the strict voter identification laws that have been passed in several states, Harold Meyerson speculated in a July column in The Washington Post about what would transpire "if Romney comes to power on the strength of racially suppressed votes."

"Mass demonstrations would be in order," Meyerson wrote. "So would a congressional refusal to confirm any of Romney's appointments. A presidency premised on a racist restriction of the franchise creates a political and constitutional crisis, and responding to it with resigned acceptance or inaction would negate America's hard-won commitment to democracy and equality."

Asked what steps they had taken to handle potential claims of illegitimacy should their candidate win the election while losing the popular vote, both camps sidestepped the question.

"Our job is to convince remaining undecideds and to turn out as many of our supporters as possible," said Obama spokesperson Ben LaBolt. "It's not to tweak our plans based on the daily poll of polls."

And Romney spokesperson Andrea Saul said, "The election isn't being held today."

According to RealClearPolitics elections analyst Sean Trende, the possibility that either candidate will win the presidency without a popular-vote majority is "pretty slim." He noted that there has been less recent polling of individual states compared with national surveys, making it likely that polls in individual states are lagging behind the broader national trends.

Still, both sides are preparing behind the scenes for an outcome that would be steeped in controversy and would almost certainly further intensify the nation's partisan divide.