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Most Americans have never heard of Virgil Goode, a former party-switching congressman with a distinctive Virginia drawl who conceivably could decide the presidential election. But he is well known to President Barack Obama's team of political advisers.

Goode served six terms in Congress from Virginia and is gathering signatures to appear on the ballot in his home state as the presidential candidate from the Constitution Party. He's already on the ballot in more than a dozen other states with an anti-immigration, pro-term limit platform he hopes makes a dent with the electorate. It's not likely to be much of a dent, but enough in Virginia for Obama campaign officials to take close notice of his potentially helpful candidacy.

Goode is one of several third-party presidential candidates who will appear on ballots across the country this fall. But within the Obama camp he is considered one of two who could tilt the race by pulling votes away from Republican challenger Mitt Romney. The other is Libertarian Party candidate Gary Johnson, a former two-term Republican governor of New Mexico whose presence on the ballot could make a difference in the presidential contest in states such as New Mexico and Colorado.

Neither candidate is considered enough of a national threat to draw comparisons to Ross Perot, whose independent campaign in 1992 attracted nearly 19 percent of the vote and whom President George H.W. Bush still blames for costing him his re-election. But Democrats see Goode and Johnson as this year's Ralph Nader, whom they still blame for Al Gore's loss to George W. Bush in 2000. Nader's liberal Green Party candidacy only attracted 2.7 percent of the national vote, but in decisive Florida, his total was greater than the 537 votes that separated Bush from Gore.

Despite that history, in most modern elections, third-party candidates haven't swayed the results, and even those who poll strongly early eventually fizzle in the end.

Still, in a national contest like this year's where Obama holds slight leads or is running virtually even with Romney in key battleground states, even a sliver of the vote in a crucial state could determine the outcome. Obama's team has scenarios whereby Obama can win states like Virginia and Colorado with less than 50 percent vote with an assist from Goode and Johnson, respectively.

That third-party candidates have become a consideration in Obama's camp illustrates one of the president's persistent challenges and his potential weakness: his inability to get above 50 percent in states he carried with some comfort in 2008.

Of all the states in play, Virginia looms among the most important. Campaigning there recently, Obama repeatedly declared that if he won Virginia he would win the election.

Virginia is even more of a keystone in Romney's strategy — one of three formerly reliable Republican presidential states that went for Obama in 2008.

Goode, who served in Congress as a conservative Democrat and then as a Republican, is running a campaign based on stopping illegal immigration and on imposing a moratorium on nearly all green cards for legal immigrants until the U.S. economy improves and unemployment falls below 5 percent. It's a stance that could either erode some of Romney's support or attract a small number of conservatives who would otherwise vote for Romney.

"If Virgil Goode gets on the ballot in Virginia that is going to make it very tough on Romney," said Democratic strategist Joe Trippi, a veteran of presidential campaigns. It's difficult to imagine Romney getting the 270 electoral votes he needs to win if he doesn't carry Virginia, Trippi said, adding that Goode "would be potentially crippling to Romney."

Romney's team downplays any damage Goode or Johnson could cause to their Republican challenger. They argue that the passion to defeat Obama among Romney supporters will outweigh any desire to vote for someone else.

"People on the right side of the spectrum are so intent on beating this president that they will see a vote for a Gary Johnson or a Virgil Goode as nothing more than a vote to re-elect Barack Obama," said Romney political director Rich Beeson.

Moreover, Beeson said, Obama might be vulnerable among disgruntled liberals who might be attracted to Johnson's libertarian social views.

"I think he's going to lose some on the left and at the end of the day you assume that that's a wash and its back to being a one-to-one race," Beeson said. "Right now it is an interesting story line, but as we get closer to the election I think these things tend to sort of fall away and you have the bulk of the electorate focused on the two primary party candidates."

A senior Obama campaign official, speaking on the condition of anonymity to discuss campaign thinking, pointed to polls that show Obama voters having greater enthusiasm for their candidate than do Romney voters. That means Romney voters could switch allegiance to a third-party candidate more easily. A recent Pew Research Center poll showed that 34 percent of Romney voters support him strongly whereas 64 percent of Obama supporters say they back him strongly.

Obama advisers say they are not doing anything to help either Goode or Johnson secure positions on the ballots of key states and that they don't plan to help their candidacies.

Johnson has encountered a challenge in Michigan from the state's Republican attorney general, who has declared him ineligible to appear on that ballot. Johnson has an innovative "Plan B": Using another Gary E. Johnson who has volunteered to have his name placed on the Libertarian Party ticket in Michigan.

Goode needs 10,000 signatures to qualify for the Virginia ballot. He has collected 14,000 signatures and is gathering more to pad his number by the state's Aug. 24 deadline. He says no Republican officials have talked to him about the race, let alone sought to discourage his presence on the ballot.

Goode and Johnson believe that in the end they will pull votes from both sides and attract independent voters who have no interest in voting for either major party candidate.

Indeed, a Pew Research Center poll in in June found that majorities of independent voters view both parties unfavorably.

"We hope to get votes from people who are dissatisfied with both," Goode said, noting that he has received encouragement from conservative Democrats as well as Republicans.

Johnson says his effect on the race depends on the state. "When you look at the polling, in New Mexico, for example, I take more votes away from Obama," he said. "In North Carolina I take more votes away from Romney. I think that's a mixed bag."

But Johnson, for one, doesn't mind the attention.

"If I were to get tagged with a spoiler role in this, that would be terrific," he said. "That would bring a lot of focus to what I'm doing."

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