Cordell Cox has volunteered for a Democrat in every presidential race since Barack Obama’s 2008 run. Next year, he might sit it out. Cox, a 33-year-old in Michigan, said he’ll probably still cast a ballot for President Biden if he’s the Democratic nominee. But, Cox said, he won’t work to increase turnout and fears that some of his friends will choose a third-party candidate or decline to vote altogether. To them, Biden’s handling of the violence in Israel and Gaza has been unacceptable. As the White House advocates for sending $14 billion in aid to Israel, Cox and his like-minded...