Fresh from ending his long-shot presidential bid, Robert F. Kennedy Jr. appeared alongside Donald J. Trump at a rally for the former president in Arizona on Friday, a potential headline-grabber that the Trump campaign hopes will help its efforts in battleground states.

Mr. Kennedy received almost a rock-star-style reception, walking onstage to fireworks, raucous cheering and the Foo Fighters song “My Hero” at an arena in Glendale, Ariz. But the political impact of his endorsement of Mr. Trump remains uncertain.

Still, Mr. Trump’s 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.”

Hours after Mr. Kennedy announced in nearby Phoenix that he was suspending his campaign and throwing his support behind Mr. Trump, he said at the rally — with Mr. Trump standing next to him — that he and the former president had found common ground.

“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, recalling a conversation he had with Mr. Trump. “Don’t you want a president that’s going to make America healthy again?”

Despite their past conflicts, Mr. Kennedy and Mr. Trump 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.

“Can you think of any time that you can look back in history and say that the people who were censoring were the good guys?” Mr. Kennedy asked. “They’re always the bad guys.”

Mr. Trump, when he retook the microphone, added: “Bobby and I will fight together to defeat the corrupt political establishment.”

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 Vice President Kamala 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.

But it was Mr. Kennedy who was the most anticipated guest.

In welcoming his endorsement on the rally stage, the Trump campaign is betting that Mr. Kennedy can convince his supporters to back the former president. 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 won’t be clear for many days or weeks. Recent polling has also indicated that Mr. Kennedy’s support overall has waned and that his supporters are less likely to vote in November than others.

What’s more, Mr. Kennedy and Mr. Trump traded savage insults earlier in the race, comments that Democrats are certain to highlight — along with the fact that Mr. Kennedy said Mr. Trump had solicited him for a role in his administration.

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. Kennedy’s endorsement came after months of attacks against him by the former president. Mr. Trump had argued that Mr. Kennedy was a liberal Democrat in disguise, but Mr. Trump has seemingly warmed up to him, and recently expressed interest in appointing him to a cabinet position if he won back the White House.

In 2017, Mr. Kennedy said that Mr. Trump had appointed him to a vaccine safety panel during the transition period before Mr. Trump was sworn in as president. Mr. Kennedy is a vaccine skeptic who has advanced widely refuted claims that vaccines cause autism. Trump’s transition team later said that no decision had been made to form the commission or put Mr. Kennedy in charge of it.

In Glendale, Mr. Trump’s supporters said they admired Mr. Kennedy and were thrilled to have him aboard.

“I think it’s awesome,” said Lani Dupre, 58, of Glendale. “He’ll be a great addition to the Trump team.”

Maggie Haberman and Chris Cameron contributed reporting.