Robert F. Kennedy Jr.’s independent campaign is over, but the question that haunts him on the campaign trail remains: Is his support coming primarily from people who might otherwise vote Republican or Democratic?

His base was small and shrinking. An average of polls from the RealClearPolitics website put him at 5%, down from the high teens of just a few months ago.

But in a close election, Kennedy voters could seal victory in November in favor of Donald Trump or Kamala Harris.

According to experts, Kennedy’s decline in electoral votes is largely due to excitement among Democrats over their new candidate, Vice President Harris.

“When Biden was running against Trump, people were looking for a third option,” said Merrill Matthews, a senior fellow at the conservative Institute for Policy Innovation who has studied the history of third-party campaigns.

“But with the arrival of Kamala Harris, who got the Democratic nomination, that changed.”

Polls reflecting independent and nonpartisan candidates — including the Green Party’s Jill Stein and left-wing activist Cornel West — have not provided a clear answer to the question of where Kennedy’s support comes from.

In some cases, his presence in the race has proven to favor Republicans, in others, Democrats. And in almost all polls, the margins are small.

However, observers believe that a significant portion of Kennedy’s left-leaning following has already returned to the Democratic Party with the rise of Harris.

Mr. Matthews said the suspension of RFK Jr.’s campaign means Trump will get a “little bit of a boost.”

“But I’m not sure it will be much, since Kennedy has been down quite a bit in the polls,” he said.

Still, a few votes in swing states could be enough to sway an election that currently appears very close. Mr. Kennedy is well aware of this, announcing on Friday that he would remove his name from the ballot only in swing states where he believes his presence would harm Trump’s campaign.

Despite being a member of the country’s most famous Democratic family, Mr. Kennedy spent most of his exit speech criticizing the party. He also spent much of his campaign clashing with the Democratic elite.

At a fundraiser this spring in the suburbs of Detroit — a key battleground in the crucial swing state of Michigan — local Democratic Party protesters stood outside with signs calling him a killjoy.

His decision to support Trump will confirm those accusations in the eyes of Kennedy’s critics.

“I think there will be a portion of his voters who will vote for Trump, and there are others who will not be satisfied with either candidate and will simply not vote,” said Melissa Smith, author of Third Parties, Outsiders, and Renegades: Modern Challenges to the Two-Party System in Presidential Elections.

But Kennedy’s cash-strapped campaign might have had more impact on the race if he had stayed or withdrawn at the last minute.

With more than two months to go until Election Day, Ms. Smith predicted his candidacy would be a “blip in history” that could quickly become old news in an election that has already produced many surprises.

But for now, Trump seems better positioned to drum up even more support for Kennedy.

The Trump campaign released a memo from pollster Tony Fabrizio saying it benefits Republicans.

“This is good news for President Trump and his campaign — pure and simple,” he wrote.

But his endorsement also carries some risk for Trump, as Democrats want to define the Republican ticket as “foreign” — meaning outside the mainstream of American politics.

Mr. Kennedy, with his extreme anti-vaccination views, could provide further material for such attacks.

Outside this week’s Democratic National Convention, one newspaper ran a tongue-in-cheek headline that it was funded by his campaign: “Kennedy Is Weird.”

It was full of anti-vaccination messages and references to conspiracy theories, with the underlying message that “weird” is good.

The Harris campaign, in a statement to RFK Jr., did not directly reference Friday’s news.

“For every American who is tired of Donald Trump looking for a new path forward, we are a campaign for you,” said Jen O’Malley Dillon, Harris’ campaign manager.