Independent presidential candidate Robert F. Kennedy Jr. has reached 15 percent or more in three approved national polls. One more, and he will have met one of CNN’s benchmarks to qualify for the debate June 27 with Presidents Biden and Trump.

But Mr. Kennedy cannot count on maintaining his current level of support as the November election nears.

It is pretty common for third-party candidates to look like they have polling momentum in the months before an election, only to come up far short at the ballot box, according to an Associated Press analysis of Gallup data going back to 1980.

That is not a sign that the polls about Mr. Kennedy are wrong right now. They just are not predictors of what will happen in the general election.

Studies have shown that people are bad at predicting their future behavior, and voting is months away. And in a year with two highly unpopular candidates in a rematch from 2020, voters may also use their early support for a third-party candidate to express their frustration with the major party choices. In the end, voters may support the candidate for whom they feel their vote can make a difference or they may decide not to vote at all.

The concept of a third party has been popular for a long time.

A poll conducted by Gallup in 1999 found two-thirds of American adults said they favored a third political party that would run candidates for president, Congress and state offices against Republicans and Democrats.

About 6 in 10 American adults have said in Gallup polling since 2013 that the Republican and Democratic parties do “such a poor job representing the American people” that a third major party is needed. In the latest Gallup polling, much of that enthusiasm is carried by independents: 75 percent say a third party is needed. About 6 in 10 Republicans and slightly fewer than half of Democrats, or 46 percent, say an alternative is necessary.

Marjorie Hershey, a professor emeritus in the political science department at Indiana University, said Americans generally like the idea of a third party until specifics emerge, such as that party’s policies and nominees.

“It’s a symbolic notion. Do I want more choices? Well, sure. Everybody always wants more choices, more ice cream choices, more fast-food choices,” Ms. Hershey said. “But if you start to get down to brass tacks and you talk about, so would it be tacos or burgers, then that’s an entirely different choice, right?”

That hypothetical support for third-party candidates often breaks down quickly.

The AP analysis looked at polling for every independent and minor party presidential candidate who received at least 3 percent of the popular vote nationally going back to the 1980 election.

In multiple elections, including the 1980, 1992, and 2016 presidential races, third-party candidates hit early polling numbers that were much higher than their ultimate vote share. For instance, in polls conducted in May and June 1980, between 21 percent and 24 percent of registered voters said they would like to see independent candidate John Anderson, a veteran Republican congressman from Illinois, win when he ran for president against President Reagan and Democratic incumbent Jimmy Carter. 

Anderson went on to earn 7 percent of the popular vote.

Part of the problem is that early polls often look quite different from the actual general election vote.

Voters “don’t know what’s going to happen between now and the election,” said Jeffrey Jones, a senior editor at Gallup. “Things are going to come up in the campaign that could change the way they think.”

Decades after Anderson, polls conducted during the 2016 presidential campaign put support for Libertarian Party candidate Gary Johnson, a former New Mexico governor, at between 5 percent and 12 percent in polls of registered voters conducted from May to July. That led some people to predict that he could do better than any third-party candidate in decades. Mr. Johnson won about 3 percent of the vote in that election.

Mr. Johnson told the AP that he believes his name should have been included in more polls, though he was in surveys used to determine eligibility for debates. He also contends that independent candidates struggle to match major party candidates in fundraising.

“It’s money, first and foremost. People don’t donate if they don’t think that you have a possibility of winning,” Mr. Johnson said. “I’m not excluding myself from that same equation. Look, am I going to give money to somebody that I know is going to lose? I’d rather go on a vacation in Kauai.”

The American electoral system makes it hard for third parties to thrive. Still, it is possible to have a significant impact without coming close to winning.

Billionaire businessman Ross Perot is among the most successful modern-day examples. He won 19 percent of the vote when he ran for president in 1992. But that was substantially lower than his support in earlier polling. In polls conducted from May to July of that year, between 30 percent and 39 percent of registered voters said they would vote for Perot.

There are already reasons to believe that at least some of Mr. Kennedy’s polling support may be a mirage. (The Kennedy campaign did not respond to a request for comment.)

A CNN poll conducted last summer when he was running for the Democratic nomination found that 2 in 10 Democrats who would consider supporting him said that their support was related to the Kennedy name or his family connections. An additional 17 percent said they did not know enough about him and wanted to learn more, while only 12 percent said it was because of support for his views and policies.

“A variable that is so different from all these other people is the Kennedy name,” said Barbara Perry, an expert in presidential studies at the University of Virginia’s Miller Center. “There’s a lot of emotion around him that I would say was not there in the Anderson, Perot, (Ralph) Nader and Johnson cases.”

There also is some evidence that Americans are using support for Mr. Kennedy to express frustration with Messrs. Biden and Trump.

Ms. Hershey notes that for many people, presidential elections can feel abstract until a few weeks before it happens, so it is good to take early poll numbers with a grain of salt.

Such polls “don’t necessarily reflect actual political issues,” Ms. Hershey said. “They reflect general views about life.”