Latinos and young people—two key Democratic voting blocs that have strayed from President Joe Biden—are among Robert F. Kennedy Jr.’s biggest supporters, according to a new poll of battleground states that comes as both Biden’s and former President Donald Trump’s campaigns have shown signs they’re unnerved by Kennedy’s influence in the race. Kennedy has the support of just 10% of voters in six battleground states in a six-way contest, but support grows to 18% for voters between the ages of...