NEW YORK - Former president Donald Trump is sharpening his attacks on 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 platform on April 26, Trump, the presumptive Republican nominee, took aim at 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,” Trump wrote.

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 April 27 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 the social media platform 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.

Trump also took a swipe at Ms Shanahan, who gave US$2 million (S$2.7 million) of the US$5.4 million that Mr Kennedy raised in March. Until 2023, she was married to Google co-founder Sergey Brin.

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!” 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…”

Trump’s barbs signalled a potential change in strategy by the former president, who Democratic allies of President Joe 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 April 24 by Quinnipiac University that showed that Mr Kennedy’s supporters were more likely to favour Trump in a two-way race. Forty-seven per cent would vote for Trump, and 29 per cent would vote for Mr Biden.

In a hypothetical five-way race that included third-party and other independent candidates, Trump and Mr Biden were deadlocked at 3 per cent, with Mr Kennedy receiving support from 16 per cent of registered voters.

Two other recent polls – by Marist College and NBC News – showed that Mr Biden performed better in multi-candidate races featuring Mr Kennedy. NYTIMES