An independent presidential candidate, Robert F. Kennedy Jr., labeled President Biden the “real spoiler” in the 2024 presidential race Wednesday — and he is calling on the incumbent president to drop out of the contest.

Mr. Kennedy’s campaign billed Wednesday’s event at Brooklyn as a “major announcement,” which fueled speculation that the attorney was dropping out of the race or launching a new ballot initiative. Instead, Mr. Kennedy gave a PowerPoint presentation of polling that shows he beats both Mr. Biden and President Trump in head-to-head matchups.

The Kennedy campaign commissioned a Zogby 50-state poll of more than 26,000 likely voters, which found Mr. Kennedy beats Mr. Biden with 367 electoral votes to 171. The poll also shows Mr. Kennedy beating Mr. Trump by 270 electoral votes to 268. In the poll, Mr. Trump beats Mr. Biden in a head-to-head matchup by 50 electoral votes.

Mr. Kennedy called on Mr. Biden to sign a “No Spoiler Pledge,” which says that each candidate agrees to co-fund a 50-state poll of 30,000 or more voters to see who is more likely to win in a head-to-head matchup against Mr. Trump. The polling must be done by mid-October, at which point the weaker candidate will agree to drop out of the race.

“To be a spoiler, a candidate has to fulfill two requirements — first, they must not be able to win themselves, and, second, by participating in the election, they must prevent someone else from winning who otherwise would,” Mr. Kennedy said.

Mr. Kennedy said that Mr. Biden does not win against Messrs. Trump or Kennedy, and so by definition he is the so-called spoiler.

“President Biden needs to do what he said he would do in 2020. He needs to fulfill his promise not to seek re-election,” Mr. Kennedy’s campaign manager, Amaryllis Fox Kennedy, said at the event. “Step aside so Bobby Kennedy Jr. can beat Donald Trump, save us from sliding back into another four years of chaos, and get our country back on track.”

Mr. Kennedy said he would sign the “No Spoiler Pledge” with Mr. Trump as well, though he clarified, “President Trump is not a spoiler because he can actually win.”

Ballot access is one of the Kennedy campaign’s biggest challenges. It so far has secured ballot access in only Utah and Michigan, and has collected enough signatures to achieve it in seven other states. The campaign also announced this week that it will be on the ballot in California under the American Independent Party.

Polls show Mr. Kennedy takes votes away from both Messrs. Biden and Trump in a three-way race. Mr. Trump has recently started to attack Mr. Kennedy, calling him a Democrat “plant.”

“’A Vote for Junior’ would essentially be a WASTED PROTEST VOTE, that could swing either way, but would only swing against the Democrats if Republicans knew the true story about him,” Mr. Trump posted to Truth Social.