Trump Turns on R.F.K. Jr. Amid Concerns He Could Attract Republican Voters

Donald J. Trump met with Union workers outside a construction site in Manhattan on Thursday.

Former President Donald J. Trump is sharpening his attacks on the independent presidential candidate Robert F. Kennedy Jr., as new polls show an overlap between their core supporters.

In a series of posts on his Truth Social media platform on Friday night, Mr. Trump, the presumptive Republican nominee, took aim at both Mr. Kennedy and his running mate, Nicole Shanahan, a wealthy Silicon Valley lawyer and investor.

“RFK Jr. is a Democrat ‘Plant,’ a Radical Left Liberal who’s been put in place in order to help Crooked Joe Biden, the Worst President in the History of the United States, get Re-Elected,” Mr. Trump wrote.

Mr. Trump, who had privately discussed the idea of Mr. Kennedy as a running mate, echoed what Democrats have been saying for months about Mr. Kennedy’s candidacy — that it could swing the election. He also appeared to be adopting a new derisive nickname for him.

“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,” he said.

Mr. Kennedy fired back on Saturday in his own social media post.

“When frightened men take to social media they risk descending into vitriol, which makes them sound unhinged,” he wrote on X. “President Trump’s rant against me is a barely coherent barrage of wild and inaccurate claims that should best be resolved in the American tradition of presidential debate.”

Mr. Kennedy further attempted to goad the former president.

“Instead of lobbing poisonous bombs from the safety of his bunker, let’s hear President Trump defend his record to me mano-a-mano by respectful, congenial debate,” he wrote.

Mr. Trump also took a swipe at Mr. Kennedy’s running mate, Ms. Shanahan, who gave $2 million out of the $5.4 million that Mr. Kennedy raised in March. Until last year, she was married to the Google co-founder Sergey Brin.

Mr. Trump referred to her as the “V.P. Candidate that nobody ever heard of” and denigrated her business credentials.

“Her business was doing surgery on her husband’s wallet!” Mr. Trump wrote in a post. “She’s more Liberal than Junior’ by far, not a serious person, and only a Pot of Cash to help get her No Chance Candidate on the Ballot … ”

Mr. Trump’s barbs signaled a potential change in strategy by the former president, who Democratic allies of President Biden and political observers have for months suggested could benefit from having Mr. Kennedy, the liberal scion, in the race.

But several recent polls have undercut such a scenario, including a survey released on Wednesday by Quinnipiac University that showed that Mr. Kennedy’s supporters were more likely to favor Mr. Trump in a two-way race. Forty-seven percent would vote for Mr. Trump, and 29 percent would vote for Mr. Biden.

In a hypothetical five-way race that included third-party and other independent candidates, Mr. Trump and Mr. Biden were deadlocked at 37 percent, with Mr. Kennedy receiving support from 16 percent of registered voters.

Two other recent polls — by Marist College and NBC News — showed that Mr. Biden performed better in multicandidate races featuring Mr. Kennedy.