Allies of Donald J. Trump are discussing ways to elevate third-party candidates in battleground states to divert votes away from President Biden, along with other covert tactics to diminish Democratic votes.

They plan to promote the independent candidate Robert F. Kennedy Jr. as a “champion for choice” to give voters for whom abortion is a top issue — and who also don’t like Mr. Biden — another option on the ballot, according to one person who is involved in the effort and who, like several others, spoke on the condition of anonymity to discuss the plans.

Trump allies also plan to amplify the progressive environmental records of Mr. Kennedy and the expected Green Party candidate, Jill Stein, in key states — contrasting their policies against the record-high oil production under Mr. Biden that has disappointed some climate activists.

A third parallel effort in Michigan is meant to diminish Democratic turnout in November by amplifying Muslim voters’ concerns about Mr. Biden’s support for Israel’s war in Gaza. Trump allies are discussing running ads in Dearborn, Mich., and other parts of the state with large Muslim populations that would thank Mr. Biden for standing with Israel, according to three people familiar with the effort, which is expected to be led by an outside group unaffiliated with the Trump campaign.

Many of these third-party-boosting efforts will probably be run out of dark-money entities that are loosely supportive of Mr. Trump. Both the Trump campaign and the main super PAC supporting the former president, MAGA Inc., are already aggressively framing Mr. Kennedy as a far-left radical to draw potential Democratic voters away from Mr. Biden.

Whatever the mechanism, the Trump team’s view is simple and is backed by public and private polling: The more candidates in the race, the better for Mr. Trump. Mr. Biden’s team agrees. And in a race that could be decided by tens of thousands of votes — as the last two presidential elections have been — even small shifts in the share of votes could change the result.

“There is no question that in a close presidential race, independent or minor party candidates can have a disproportionately large impact,” said Roger Stone, who is Mr. Trump’s longest-serving political adviser and who has worked on third-party campaigns, including advising Gary Johnson, the Libertarian Party’s nominee in 2012.

Republican donors are pouring funds into Mr. Kennedy’s independent bid for the presidency. He has raised substantially more from donors who previously supported Mr. Trump than he has from those who backed Mr. Biden. Some are big names in Republican politics who have so far given relatively small amounts, including $3,300 last August from Elizabeth Uihlein, whose family is among the G.O.P.’s biggest contributors.

Timothy Mellon, the largest single donor to Mr. Kennedy’s biggest super PAC, is also the largest backer of MAGA Inc. Mr. Mellon, a reclusive billionaire from one of America’s wealthiest families, has over the past year given the Kennedy super PAC $20 million and the Trump super PAC $15 million, as of the most recent disclosures that were filed in March. Another prominent Kennedy backer is Patrick Byrne, the former chief executive of Overstock.com who worked with Mr. Trump on his effort to overturn the 2020 election.

Mr. Trump himself is intensely interested in the third-party candidates, according to aides. He is eager to know what their effect is expected to be on the race and how they are polling, although his engagement beyond asking questions of those around him is unclear.

Mr. Trump has been worried about the Libertarian Party pulling conservative voters away from him in November. But Richard Grenell, who is the former acting director of national intelligence and who is expected to play a big role in any second Trump administration, has been using his connections with Libertarian activists and donors to try to persuade them to attack Mr. Biden more than Mr. Trump, according to people familiar with his efforts.

Other Trump supporters are trying to help third-party and independent candidates with the expensive and arduous process of gathering the signatures needed to get on state ballots. Scott Presler, the conservative activist whom Lara Trump said she wanted as an early hire at the Republican National Committee, publicly reached out on social media to Ms. Stein and Cornel West, a left-wing academic who is running for president as an independent, to offer his help in collecting signatures to get them on the ballot.

Mr. Presler could not be reached for comment.

The moves by Trump allies come as the Democratic Party, alarmed by the potential for third-party candidates to swing the election, has mobilized a team of lawyers to scrutinize outsider candidates, including looking into whether they’ve followed the rules to get on state ballots.

For decades, third-party candidacies have loomed large in U.S. presidential elections. The best known in modern history is Ross Perot, whose run as a billionaire populist independent in 1992 garnered 19 percent of the vote and helped Bill Clinton win with only 43 percent of the popular vote. Ralph Nader, a Green Party candidate, siphoned votes away from Vice President Al Gore in the nail-biter 2000 presidential race against George W. Bush.

And in 2016, Ms. Stein, as the Green Party candidate, gave a meaningful — and arguably election-deciding — boost to Mr. Trump by drawing progressive voters away from former Secretary of State Hillary Clinton in Pennsylvania, Michigan and Wisconsin. That year, the billionaire businessman and Home Depot co-founder Bernie Marcus, a supporter of Mr. Trump, helped fund efforts to bolster Ms. Stein.

Polling shows that third-party candidates could play an especially large role in 2024. Most Americans are unhappy with the choice between Mr. Trump and Mr. Biden. Voters are increasingly disillusioned with the two major parties, and trust in American institutions has eroded over the past 30 years. Those trends provide an opening for candidates who style themselves as anti-establishment outsiders willing to blow up the system. Mr. Trump took advantage of similar conditions in 2016.

In a Quinnipiac University poll in late March, Mr. Biden and Mr. Trump both received less than 40 percent of the vote in a hypothetical five-way race, with Mr. Kennedy getting 13 percent, Ms. Stein receiving 4 percent and Mr. West capturing 3 percent.

In the multicandidate race, Mr. Trump led by a single percentage point; Mr. Biden led Mr. Trump by three percentage points in a hypothetical head-to-head race.

“The path to victory here is clearly maximizing the reach of these left-wing alternatives,” said Stephen K. Bannon, the former White House chief strategist who also served as Mr. Trump’s campaign chairman in 2016.

“No Republican knows that oil production under Biden is higher than ever. But Jill Stein’s people do,” added Mr. Bannon. “Stein is furious about the oil drilling. The college kids are furious about it. The more exposure these guys get, the better it is for us.”

Brian Hughes, a spokesman for Mr. Trump, described Mr. Kennedy as a “leftist and liberal with a history of supporting an extreme environmental agenda.” He said more broadly of the Democratic push to challenge outsider candidates, “While Joe Biden and his allies claim to defend democracy, they are using financial and legal resources to prevent candidates access to the ballot.”

“President Trump believes any candidate who qualifies for the ballot should be allowed to make their case to America’s voters,” he added.

For months, the Trump team has been privately polling various iterations of third-party tickets in battleground states. It has concluded that candidates floated for the Green Party and No Labels, which recently abandoned its effort to field a presidential candidate, pulled substantially more votes from Mr. Biden than from Mr. Trump.

A person briefed on other polling by Trump allies said that while it varies by state, Mr. Kennedy also pulls more votes from Mr. Biden than from Mr. Trump. The person cited as an example the Trump team’s recent private polling of voters in Arizona. Mr. Trump loses Hispanic voters by a close margin in a head-to-head contest against Mr. Biden there, but he wins Hispanic voters on the full ballot in Arizona — an indication that third-party candidates draw more heavily from Mr. Biden’s core constituencies than from Mr. Trump’s.

Still, Mr. Kennedy is seen as more of a potential threat to Mr. Trump. He has spent the past few years appearing on conservative news media programs and talking about issues like his fierce opposition to the Covid-19 vaccine. Advisers to Mr. Trump say that many Republican voters don’t know anything about Mr. Kennedy’s liberal views on gun control and the environment, and the Trump team hopes to bring back some of those voters after framing Mr. Kennedy as a liberal Democrat.

Allies of Mr. Trump and Mr. Biden are in a tug of war to define Mr. Kennedy, who has far more support than any other third-party candidate.

Democratic lawyers and operatives, many of whom have privately said that neither Mr. Gore nor Mrs. Clinton had teams that took third-party candidates seriously enough, are fighting hard to keep Mr. Kennedy off the ballot. The Democratic National Committee hired Lis Smith, a veteran communications operative, and tasked her with branding Mr. Kennedy as a pro-Trump spoiler candidate.

Mr. Kennedy’s campaign and the super PACs backing him have paid an array of lawyers and consultants to secure ballot access. One of the consultants, Rita Palma, was captured in a video detailing a strategy to encourage New York voters to support Mr. Kennedy: “The Kennedy voter and the Trump voter, our mutual enemy is Biden.” Ms. Palma outlined a hypothetical scenario in which Mr. Kennedy would win enough electoral votes to prevent either Mr. Trump or Mr. Biden from winning 270 electoral votes, pushing the decision to Congress in what is known as a contingent election.

On her X account, Ms. Palma has expressed support over the years for both Mr. Kennedy and Mr. Trump. In posts first reported by CNN on Tuesday, she had endorsed Mr. Trump’s false claims that the 2020 election was stolen and described Sidney K. Powell, who has pleaded guilty to six misdemeanor counts related to Mr. Trump’s efforts to overturn his 2020 election loss in Georgia, as “My person of the decade.”

Stefanie Spear, a spokeswoman for the Kennedy campaign, described Ms. Palma as “a ballot-access consultant” for upcoming signature collection efforts in New York. Of Ms. Palma’s remarks about the hypothetical scenario, Ms. Spear said Ms. Palma’s statements “in no way reflect the strategy of the Kennedy campaign.”

Ms. Spear did not respond to requests for comment about the Trump allies’ efforts to elevate Mr. Kennedy, or to inquiries about Ms. Palma’s support for Mr. Trump’s claims about the 2020 election.

Many conservative news media personalities and influencers recently turned against Mr. Kennedy after he decided to run as an independent instead of as a Democrat and it became apparent that he could pull votes from Mr. Trump.

Still, one complication with attacking Mr. Kennedy is that Mr. Trump has made clear that he likes him.

Mr. Trump put out a statement on Truth Social, his social media platform, that called Mr. Kennedy “a radical-left Democrat,” but he has mostly laid off him otherwise. Mr. Trump has called Mr. Kennedy a “very smart person” and has even privately floated him as a potential running mate, though his advisers view that prospect as extremely unlikely.

An outside group aligned with Mr. Trump asked a question about a Trump-Kennedy ticket in a poll several weeks ago, according to a person with knowledge of the survey. The results were not particularly striking. Mr. Trump had told an ally that he believed Mr. Kennedy could help him with voters who were upset with him for his support of the Covid-19 vaccine.

“I like Trump-Kennedy. I like the way that sounds,” Mr. Trump told another ally recently. “There’s something about that that I like.”

Ruth Igielnik contributed reporting.