The Democratic presidential candidate Robert F. Kennedy Jr. came to Capitol Hill on Thursday and pointedly declared that he is neither an antisemite nor a racist, while giving a fiery defense of free speech and accusing the Biden administration and his political opponents of trying to silence him. Mr. Kennedy, an environmental lawyer who turned to anti-vaccine activism and has trafficked in conspiracy theories, was referring to the storm that erupted after The New York Post published a video in which he told a private audience that Covid-19 “attacks certain races disproportionately” and may have been “ethnically targeted” to do...