Meanwhile, Romney has tried to heal fractures between Paul loyalists and the Republican old guard by deploying his former rival's son, Sen. Rand Paul of Kentucky, to campaign events.
New Hampshire state Sen. Andy Sanborn, an adviser to the elder Paul, said Johnson could score with voters at Romney's expense.
"That type of a libertarian candidate will always do well here. I'm hoping frankly that the race isn't close enough that Mr. Johnson will have a material impact in it," said Sanborn, who said he planned to vote for Romney.
Johnson considers himself a headache for both Obama and Romney.
"I'm more conservative than Romney on dollars and cents. I'm more liberal than Obama when it comes to social issues," he said in an interview with The Associated Press.
Johnson's support for gay marriage, eased immigration and a scaling back of government search powers authorized after the Sept. 11 terror attacks make him a wild card in some key states.
One is North Carolina, where Obama prevailed in 2008 by a slim 14,000 votes. Some 40,000 votes were cast for minor party candidates or write-ins, with Libertarian Party nominee Bob Barr getting most of them. Michael Munger, the party's nominee for North Carolina governor the same year, doubted Johnson would have as much of a one-sided effect as the conservative hard-liner Barr.
"I actually think there's sort of a gentleman's agreement about Gary Johnson that neither party brings him up because it takes votes from both sides. In 2008, the Democrats mentioned Bob Barr," Munger said. "They worked to remind people of the fact Bob Barr was in it and real conservatives might want to consider him."
In Colorado, Johnson has aligned himself with a ballot measure to legalize marijuana for recreational use.
Democratic strategist Rick Ridder, a Denver-based veteran of presidential campaigns, said some Democratic activists are supporting Johnson because of his stance on the referendum. But he thinks most voters passionate about making the drug legal to possess will send a message through the proposition itself and make other calculations on the presidential race.
Goode is also on Colorado's ballot. That's notable because two years ago former Colorado Rep. Tom Tancredo finished second in the race for governor under the American Constitution Party's banner.
Johnson, at least, has no problems being labeled a possible spoiler.
"A wasted vote is voting for someone you don't believe in," he said. "Vote for someone you believe in because that's how you change politics."