COLUMBIA, S.C. >> Donald Trump is known for leveling constant and often personal attacks on top rivals such as Joe Biden. Lately, he’s increasingly taking that same approach against independent presidential candidate Robert F. Kennedy Jr.

Among the recent jabs, Trump this past week posted a roughly four-minute video online in which he called Kennedy “fake,” a “Democrat ‘Plant’” and “Radical Left Liberal who’s been put in place” to help the Democratic president. Trump railed against Kennedy’s family as “a bunch of lunatics.”

“He is not a Republican so don’t think you’re going to vote for him and feel good,” the former president and presumptive Republican nominee told supporters in the Truth Social post.

Directing such fierce attacks at Kennedy may signal concern from Trump and his campaign about the independent’s bid in what’s expected to be a tight November election, when a third-party hopeful siphoning even a small amount of support could sink one of the major candidates.

Six months out from an Election Day in which many Americans have voiced their dissatisfaction at a rematch between Trump and Biden, Kennedy has been offering himself as an alternative. Some of the issues Kennedy focuses on — stalwart support for Israel and criticism over COVID-19 lockdowns — could appeal more to conservative voters than Democrats.

Polls at this point show far more Republicans than Democrats have a favorable opinion of Kennedy, although many Americans don’t know who he is. A February Associated Press-NORC Center for Public Affairs Research poll found that about half of Republicans, 53%, had a favorable view of him, compared with 30% of Democrats. About one-quarter in each case said they didn’t know enough about Kennedy to say.

Kennedy’s campaign argues that he threatens both Trump and Biden, who boasts support from several members of Kennedy’s own family and called the endorsements “an incredible honor.” The president has largely ignored Kennedy, who previously challenged him for the Democratic nomination before launching an independent bid.

Kennedy has gone after Trump as well, challenging him to a debate when both men speak — on separate days — at the Libertarian Party convention later this month. Kennedy claims Trump’s backers are “wavering” in their support.

But Kennedy faces steep challenges.

As an independent candidate, his name appearing on ballots isn’t automatic. He has had to work to secure ballot access across the 50 states, a process that Kennedy has said will be completed by this summer. According to his campaign, he has achieved that marker in five states — California, Delaware, Michigan, Oklahoma and Utah — with enough signatures collected for eight others. Officials haven’t verified those numbers in some states.

Kennedy has argued that his fairly strong showing in a few national polls gives him a reason to consider himself competitive, though horse-race polls are generally unreliable this far out from an election. This isn’t a new trend for third-party candidates in presidential elections. During the 2016 campaign, early national polls showed libertarian Gary Johnson’s support in the high single or low double digits; he ultimately received only about 3% of the popular vote.

Supporters flocking to Kennedy’s events, including a recent comedy showcase in a Detroit suburb, describe themselves as coming from across the political spectrum, from those who traditionally back third-party presidential efforts to disaffected Democratic and Republican voters. That included those who have previously backed both Biden and Trump, but are either jaded by or unenthusiastic about them now.

Ben Carter, a registered nurse from White Lake, Michigan, said that he supported Trump in 2016 but “couldn’t do it again,” opting for Biden four years later. This year, Carter said he admired Kennedy’s willingness to take on difficult topics, seeing the independent candidate as willing to voice unpopular opinions but doing so in a way more palatable than Trump.

“I just don’t hear Kennedy going out, lying about things. Trump, he just stands up in front of the camera and baldfaced lies about stuff that we know are true,” Carter said. “He has his opinions that you might not agree with, but I haven’t seen him stand up in front of a crowd and just lie to people.”

Trump’s supporters admit they are curious about Kennedy’s bid, even if they remain fiercely loyal to Trump.

“He’s super interesting,” Kim Hanson, a financial consultant from Hartford, Wisconsin, said on the sidelines of Trump’s recent rally in Waukesha, Wisconsin. “I love hearing from him.”

But Hanson, a Trump supporter, said she worried that the novelty appeal of voting for Kennedy could detract from Trump’s support.

“I am concerned about people voting for people they think aren’t going to get in, and they aren’t voting for Trump,” she said.

There are some issue areas where Kennedy and Trump seem aligned.

Like Trump, Kennedy has been a fierce defender of Israel in its war with Hamas. In April, he suggested that the prosecution of rioters who violently attacked the U.S. Capitol on Jan. 6, 2021, might be politically motivated, partly aligning himself with the false portrayal being pushed by Trump and his allies.

Kennedy levied some criticism on Trump, saying the attack on the Capitol happened with Trump’s “encouragement” and “in the context of his delusion that the election was stolen from him.” But Kennedy also said that as president, he would appoint a special counsel to examine whether Trump allies were unfairly singled out for prosecution.