By running as the Libertarian Party candidate, Johnson hoped to gain the support of the relatively small but fiercely loyal following of Ron Paul, the Texas congressman who has failed in three bids to win the Republican presidential nomination. So far, the polling suggests little progress by Johnson in winning over Paul's supporters.

An NBC News/Wall Street Journal/Marist survey earlier this month in Virginia showed 1% of likely voters -- about 38,000 based on the 2008 turnout in the state -- supported an unspecified candidate other than Obama or Romney. The bulk of that support appeared to be from strongly liberal or moderate younger voters, a demographic closer to Obama's base.

Obama could get hurt by two other minor party candidates -- Jill Stein of the Green Party and Rocky Anderson of the Justice Party -- but both are considered far less likely to get enough support to undermine the president.

A CNN/ORC national poll at the end of September showed Stein with 3% compared to 4% for Johnson, while Goode and Anderson had negligible support. However, Stein failed to register in CNN/ORC polls in recent weeks in Florida and Nevada, and she got 1% support in Ohio compared to 3% for Johnson.

According to Schiller, possible voter apathy by liberals disappointed with Obama's first term helped motivate the president's campaign to mount a huge effort to boost turnout both in early voting and on Election Day.

"I think they understood they had to get the people who would definitely vote for Obama to the polls, period, with no wavering and no indecision," she said.

CNN's Holland warned against assuming supporters of third party candidates changed their mind from a major party contender.

"Minor party supporters usually fall into that category because they don't like the two major party candidates," he said. "So in a hypothetical world in which the race were only between the two major party candidates, a lot of minor party voters would have just stayed at home."