With a month to go before what is widely expected to be an extraordinarily close election, an extra element of unpredictability looms: In every battleground state, there is at least one third-party or independent presidential candidate on the ballot.

None of these candidates will come anywhere close to winning the presidency. Jill Stein, the Green Party nominee, is polling at about 1 percent nationally, according to New York Times polling released last week. Same with Chase Oliver, the Libertarian Party candidate.

But their presence on the ballot in just a few states could take just enough votes away from the major party candidates to tip the balance of the Electoral College and the election, in what is known as the “spoiler” effect.

To be a spoiler in the election, a third-party candidate would “either have to have a large amount of support, or the election has to be remarkably close,” said Bernard Tamas, a professor of political science at Valdosta State University who has written a book on third parties in U.S. politics.

“This may wind up being very, very close in these key swing states,” he said.

The Democratic Party and its allies have been more openly concerned than Republicans, in recent years, about the impact of third-party candidates, and this election is no exception. As they see it, every vote that is not for Vice President Kamala Harris is a vote for former President Donald J. Trump.

“The presidential race could very well come down to a few thousand votes in a handful of states,” said Rahna Epting, the executive director of MoveOn, a liberal activist group. “These third-party candidates — they still remain a big threat this cycle.”