WASHINGTON — 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 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 already is 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 notice of his 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.

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 attracted only 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 strong early eventually fizzle.

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 his persistent challenges and his potential weakness: his inability to get above 50 percent in states he carried with some comfort in 2008.