One of the most attention-grabbing days of Robert F. Kennedy Jr.’s independent presidential bid was also its last.
After toiling for months as an electoral afterthought, Mr. Kennedy suspended his long-shot campaign on Friday and endorsed former President Donald J. Trump in a speech in Phoenix carried live by television networks. Then, he traveled across town to speak in front of the largest rally audience since he began his third-party run last year: an audience of 17,000 at a Trump event at an arena in Glendale, Ariz.
As he shook hands with Mr. Trump amid bursts of fireworks, Mr. Kennedy was, briefly, the star of the show, a new attraction for the Trump campaign. But it was unclear what impact, if any, Mr. Kennedy’s endorsement of Mr. Trump would have on the 2024 race.
Framing his third-party bid as an outsider movement and a breath of fresh air for Americans fed up with partisan politics, Mr. Kennedy initially attracted significant support — more than 20 percent in some early polls — and was especially popular with Hispanic voters. Many voters had said they were frustrated with the lack of choice between two unpopular and familiar candidates: Mr. Trump and President Biden.
But Mr. Kennedy had been falling recently in polls, and plummeted further after Vice President Kamala Harris took the mantle of Democratic nominee from Mr. Biden, luring some wayward Democrats back home. Even those supporters who have remained steadfast to Mr. Kennedy are less likely than others to say they will vote in November, and polls have not provided a consistent answer as to whether Mr. Kennedy’s supporters would prefer Ms. Harris or Mr. Trump.
Still, Mr. Trump and his allies on Friday relished the fact that the former president had won the backing of a member of America’s most storied Democratic family, albeit one who has had many of his relatives denounce him and his endorsement of Mr. Trump. Of all the outlandish political news stories of the summer, mused Charlie Kirk, the founder of Turning Point USA, which helped organize the rally, “maybe most remarkable of all: A Kennedy has endorsed a Republican.”
Mr. Trump predicted that Mr. Kennedy was “going to have a huge influence on this campaign,” promising that “Bobby and I will fight together to defeat the corrupt political establishment.”
But it is hard to know whether he will make a dent in the larger problem Mr. Trump has newly faced: regaining the spotlight and the narrative after several weeks of momentum for Ms. Harris.
As the vice president has surged ahead in fund-raising and tightened the race, according to polls, Mr. Trump has bemoaned the ouster of Mr. Biden, whom his allies viewed as an easier opponent, held a series of rambling news conferences and offered a freewheeling rebuttal on Fox News to Ms. Harris’s acceptance speech on Thursday night.
Mr. Trump has traded barbs with Mr. Kennedy in the past, but they have similar grievances that they could easily weave together on the campaign trail. They both blame a shadowy, bureaucratic deep state for many of the nation’s ills, and they argue that technology companies and Democrats want to suppress free speech.
“We talked not about the things that separated us — because we don’t agree on everything — but on the values and the issues that bind us together,” Mr. Kennedy told the crowd in Glendale, recalling a previous conversation he had with Mr. Trump. “Don’t you want a president that’s going to make America healthy again?”
Mr. Trump’s campaign stop in Glendale was his final event in a five-day swing through battleground states, timed to coincide with the Democratic National Convention in Chicago in an attempt to avoid ceding the spotlight to Ms. Harris.
For decades, Arizona was a reliably conservative state, but Democrats have taken advantage of Republican infighting in recent years to capture statewide offices, including for governor and both U.S. Senate seats. Joseph R. Biden Jr., then a presidential candidate, turned Arizona blue in 2020, but the state appeared to be trending back toward Mr. Trump earlier this year, as voters expressed concern about Mr. Biden’s age and the direction of the country.
Ms. Harris, though, has revitalized the Democratic base and made the state competitive again. Recent polls suggest a deadlocked race, and the party showcased its deep bench of prominent Arizona supporters at its convention, with speeches by Senator Mark Kelly, former Representative Gabrielle Giffords and others.
Republicans offered a rebuttal of sorts at Mr. Trump’s rally on Friday, featuring a litany of big names of their own, including Representatives Paul Gosar, Eli Crane and Andy Biggs. Another speaker was Kari Lake, a prominent Trump ally who is running for Senate and who became a leading proponent of his false stolen-election claims.
The choice of venue was another retort to Democrats. Ms. Harris rallied at the same location, Desert Diamond Arena, several weeks ago, and her campaign said the crowd numbered 15,000 people. Turning Point said the crowd in the arena on Friday was 17,000. Mr. Trump has flinched at Ms. Harris’s crowd sizes and falsely claimed that she was using artificial intelligence to inflate them in photos.
In his speech, Mr. Trump again broke with advisers who have urged him to focus on policy rather than veering into personal attacks. In recent weeks, he has taken events billed as opportunities to discuss components of his platform and set off on wide-ranging tangents filled with insults of Democrats.
“You say, ‘Don’t get personal.’ I have to get personal,” Mr. Trump said on Friday, proceeding to launch insults against the Obamas, Ms. Harris, Mr. Biden and Representative Ruben Gallego of Arizona, now a U.S. Senate candidate, whom he called a “maniac” and a “loser.” In fact, the only non-Republican he praised was Mr. Kennedy, whom he had once called a “Democrat plant” and a “radical left liberal” — to which Mr. Kennedy had responded that Mr. Trump was a “frightened” man who sounded “unhinged.”
But when Mr. Trump shared the stage with his former rival, those spars went unmentioned.
In welcoming his endorsement on the rally stage, the Trump campaign is betting that Mr. Kennedy can bring his supporters with him. In a memo earlier on Friday, the Trump campaign’s lead pollster, Tony Fabrizio, described the end of Mr. Kennedy’s candidacy as a clear benefit to the Republican nominee.
“This is good news for President Trump and his campaign — plain and simple,” Mr. Fabrizio wrote, describing a majority of Mr. Kennedy’s voters as overwhelmingly breaking in Mr. Trump’s favor.
Just how significant those percentages will be remains to be seen. The polling that exists about where Mr. Kennedy’s voters might go is based on the hypothetical scenario of his leaving the race. The actual impact of his departure will not be clear for many days or weeks.
Onstage on Friday, Mr. Trump renewed, “in honor of Bobby,” an unfinished pledge from his first term to create a commission that would release the remaining sealed files related to the assassination of John F. Kennedy, Mr. Kennedy’s uncle. Less than 1 percent of the records remain sealed, according to the Central Intelligence Agency.
Mr. Trump, who has said he would pardon those who rioted at the Capitol on Jan. 6, 2021, suggested at the rally that his supporters needed to take the country back from Democrats. “We have to take our country away from these people that are going to destroy our country,” he told the audience.
Earlier in the day in Phoenix, at his speech announcing the suspension of his campaign, Mr. Kennedy said Mr. Trump had offered him a role in a second Trump administration, dealing with health care and food and drug policy. In Glendale, Mr. Trump said that, if elected to a second term, a panel of experts “working with Bobby” would investigate obesity rates and other chronic health issues in the United States.
Mr. Kennedy later said was “choosing to believe” that “this time” Mr. Trump would honor the agreement. During the transition period before Mr. Trump took office in 2017, Mr. Kennedy said Mr. Trump had offered him a spot on a vaccine safety commission, only to have the president-elect’s team distance itself from such claims hours later.
Mr. Kennedy’s remarks on Friday sought to cement what he saw as a legacy: pitching his policies on food, health and the environment and railing against the Democratic Party, which he believed treated him unfairly.
Maggie Haberman and Chris Cameron contributed reporting.