The likelihood of a third-party U.S. presidential candidate becoming a dominant figure who can influence the 2012 presidential election is fading. So far no one has burst onto the national campaign scene as did billionaire businessman Ross Perot, who ran for president in 1992 and 1996, or consumer protection activist Ralph Nader, who ran as an independent candidate in 2004 and 2008, but is best known as the Green Party nominee in 2000 who acted as a spoiler and tilted the election toward George W. Bush and away from Al Gore, something Nader disputes. Nader also was a Green Party candidate in 1996. The presence, or absence, of a third-party candidate could be an advantage for either President Obama or presumptive Republican nominee Mitt Romney, depending on the point of view. "The absence of a viable third-party candidate has a huge impact on the presidential race," said commentator Steven Schier, a political science professor at Carleton College in Minnesota. "Previous polling showed that when possible, third-party candidates like New York City Mayor Michael Bloomberg were added to the horse race question, President Obama increased his lead." Why? "[Because] with a third-party candidate, voters opposed to Obama split their vote, putting Romney further behind," Schier explained. "So the absence of a third-party candidate is a big boost to Romney." Obama has unified his party coalition, which worked well for him when he had two potential opponents, Schier said. "Romney, for his part, benefits greatly by being the only serious alternative on the ballot to Obama," he said. "His position as the only alternative is what makes a Romney victory in 2012 possible." Third-party candidates -- whether on the Libertarian ticket, the Being Human ticket or the American Beer Drinkers ticket -- have a tough time finding a level playing field because each state controls how its ballot is accessed. "Each state has its own mountain of minutiae," physician Jill Stein, presumptive Green Party presidential nominee, told USA Today recently. "There are different people to work with, different procedures in each state." (The Green Party convention ends Sunday.) For instance, voters in Colorado and New Mexico can vote for Virgil Goode of the Constitution Party or Libertarian Gary Johnson or several other third-party candidates, but Oklahomans cannot, USA Today reported. "The time, money and energy spent getting on the ballot is more than the time, money and energy spent once we are on the ballot in most of these states," said Phil Huckleberry, co-chair of the ballot access committee for the Green Party. Project Vote Smart, which provides information on political candidates and their positions, tracks more than 300 people who have either declared their presidential candidacies or are potential "draft" or write-in candidates, USA Today said. But, Director Kristen Vicedomini told the newspaper, "probably under 15 would get on any presidential ballot ... [and] not even some of the major third parties will get on all 50." Some states require only the payment of a filing fee and an affidavit of intent while others require the collection of verified signatures -- usually numbering in the thousands. "Republicans and Democrats have been very successful in making it virtually impossible without the expenditures of many millions of dollars to get on the ballots of every state," said Rocky Anderson, a former Salt Lake City mayor running as the candidate of the newly formed Justice Party. Even so, U.S. registered voters seem to have taken a ho-hum attitude about third-party candidates this year, a recent Gallup poll indicated. The June 7-10 Gallup poll asked a special presidential preference question, listing three third-party candidates in addition to Obama and Romney. Libertarian Johnson was the choice of 3 percent of registered voters and the Green Party's Stein, the choice of 1 percent, while 2 percent offered Ron Paul, the U.S. representative from Texas who has challenged Romney in the primaries and is working to collect delegates before the Republican National Convention in August. One percent mentioned someone other than the listed options. Paul would face a near-impossible task of mounting a third-party run because of the myriad of state laws that prevent him from being on the ballot because he failed to secure the nomination of his preferred party, said Paul supporter Robin Koerner, publisher of WatchingAmerica.com and a political commentator for the Huffington Post and the Moderate Voice. "The only possibility would be if someone already on a third-party ticket stepped aside for him to take over their ticket. Also, that party would need to have ballot access in 50 states already secured," Koerner said. "The only party that satisfies that criterion is the Libertarian Party, but they have already chosen Johnson as their guy -- so I think this is something that cannot be done." After the convention, Paul backers should, among other things, do "exactly what [they were] doing the day before the convention," Koerner wrote recently for the Huffington Post. "Paul has said consistently that it's not about him, the man, but about the message he carries. It's not about his campaign, but about the country he loves." "Apart from everything else, then, we do a disservice not only to [the] Liberty [movement], but also to the man [Paul], if we let the end of his campaign translate into a dip in our efforts to spread the message until it has infected our entire political system," Koerner wrote. With no high-profile third-party candidate entering the presidential race so far, the third-party vote for president likely would be limited this cycle, Gallup said. At most, 5 percent to 7 percent of registered voters say they would vote for someone other than Obama or Romney. Results are based on telephone interviews with 899 registered voters conducted June 7-10 and has an overall margin of error of 4 percentage points.