Democrats say they have a simple plan for discrediting Robert F. Kennedy Jr. now that he has dropped out of the race and endorsed former President Donald J. Trump: calling him “weird.”

It’s a playground-style strategy that the Harris campaign has already deployed against Mr. Trump and his running mate, Senator JD Vance of Ohio, and the put-downs have proved popular with the Democratic base.

The party doesn’t have to look far for ammunition. Mr. Kennedy has recently made headlines for decapitating a dead whale with a chain saw and strapping its head to the family minivan, as well as dumping a deceased bear cub in Central Park. Now the Democrats want to attach his baggage to Mr. Trump.

On Friday, the Democratic National Committee unveiled billboards near the rally in Arizona where Mr. Kennedy, whose support once reached as high as 20 percent in some polls, endorsed the former president. The billboards showed an image of Mr. Trump flanked by Mr. Kennedy and Mr. Vance, with the text “Weird As Hell” in large type.

Democrats and their allies also plan to try to make Mr. Trump own some of Mr. Kennedy’s fringe positions and conspiratorial views, including his false statements about the dangers of vaccines and his brief call for a national abortion ban.

“A weirdo campaign just got weirder,” said Matt Bennett, a co-founder of Third Way, a Democratic think tank that has led efforts to stop independent and third-party candidates from siphoning votes from Democrats. “This campaign of freaks is not going to do Republicans any favors.”

And on Monday, after Tulsi Gabbard, the former Democratic congresswoman from Hawaii who has rebranded herself as a celebrity in the MAGA movement, endorsed Mr. Trump, the D.N.C. issued a news release with the headline: “Trump’s Circle of Weirdos Gets Even More Extreme.”

For months, Democrats worried that Mr. Kennedy’s third-party candidacy would cost them crucial swing voters in a close race against Mr. Trump.

But those fears eased as Mr. Kennedy’s support declined, and they have nearly evaporated since he endorsed Mr. Trump. Democrats believe that Mr. Kennedy’s exit from the race plays into their longstanding argument that he is essentially a right-wing stooge and that it bolsters their case that voters should see the election as a binary choice between Mr. Trump and Vice President Kamala Harris.

And they say Kennedy voters are too small in number and disconnected from politics to have anything more than a marginal impact in November.

“Democrats have always wanted this race to be a choice: a choice between Trump and Harris,” Mr. Bennet said. “Do they want four more years of Trump and all of his chaos, or do they want Harris? And giving them other options really makes that choice less clear.”

But Republicans point to Democrats’ efforts to weaken Mr. Kennedy — including support for legal challenges to his ballot access in several states and holding an event where Kennedy relatives repudiated him — as evidence that they are worried he could influence the race.

Tony Fabrizio, a pollster for the Trump campaign, wrote in a memo last week that Kennedy voters in the top seven battleground states could break for Mr. Trump over Ms. Harris in numbers large enough to sway the final outcome in a tight race.

“The Harris campaign is whining because they’re losing,” said Karoline Leavitt, a spokeswoman for the Trump campaign. “And they know R.F.K. Jr. joining the winning team hurts their chances of defeating President Trump even more.”

For its part, the Harris campaign has adopted a conciliatory approach toward former Kennedy voters. Ms. Harris’s team has previously reached out to disaffected Republicans who voted for former Gov. Nikki Haley of South Carolina in her primary race against Mr. Trump.

“Vice President Harris wants to earn your support,” Jen O’Malley Dillon, the Harris campaign chair, said in a statement about Mr. Kennedy’s departure from the race.

To many Democrats, it was hardly a surprise when Mr. Kennedy, who did not immediately respond to a request for comment, endorsed Mr. Trump. The two had been playing political footsie for weeks, exchanging phone calls and negotiating behind the scenes. Last week, the former president said he would consider his former rival for a position in his administration. And the largest donor to a super PAC supporting Mr. Kennedy was the billionaire Timothy Mellon, who has also contributed extensively to Mr. Trump.

Mr. Kennedy’s campaign has been seriously weakened in recent weeks.

Polling showed that his support from voters had fallen significantly over the last few months, with Democrats taking credit for messaging that drew attention to his record. That trend only continued when Ms. Harris replaced President Biden on the ticket. Some Kennedy supporters were so-called double haters — voters who disliked both Mr. Biden and Mr. Trump. His fund-raising had also all but dried up.

Before he dropped out, Mr. Kennedy was polling worse than Gary Johnson had in 2016 when he ran on the Libertarian ticket.

“Once around 15 percent, R.F.K. Jr.’s support has been in free fall — now under 5 percent — and he’s not positioned to deliver any electoral benefit to Trump,” Ramsey Reid, the Democratic National Committee’s campaign manager for its effort against third-party and independent candidates, wrote in a memo.

And many of Mr. Kennedy’s supporters were low-information voters with little track record of showing up on Election Day, meaning their support for any candidate was not guaranteed.

Rahna Epting, the executive director of the liberal activist group MoveOn — which invested heavily in attacking Mr. Kennedy — said he had gone from being a “dangerous” spoiler to a nonentity.

“He’s irrelevant,” Ms. Epting said, calling his campaign “opportunistic” and saying it was “solely driven to damage the Democrats’ chances of winning the White House,” as evidenced by his endorsement of Mr. Trump.

On Sunday, it also became clear how Republicans could be forced to defend Mr. Kennedy’s controversial views.

In an interview on NBC’s “Meet the Press,” Mr. Vance was asked about Mr. Kennedy’s statements about vaccines, antidepressants and the Sept. 11 attacks. The Ohio senator hedged and turned to attacking Ms. Harris before the host, Kristen Welker of NBC News, asked him if he agreed with Mr. Kennedy’s statement that he would not “take sides” on what happened on Sept. 11.

“Well, of course I don’t agree with that, Kristen,” Mr. Vance replied. “Now, to be clear, I don’t know what R.F.K. actually said there. Maybe there was additional context.”

Rebecca Davis O’Brien contributed reporting from New York.