As he celebrated victory early Friday after winning a parliamentary election, George Galloway, a veteran left-wing firebrand, directed his attack squarely at the leader of Britain’s opposition Labour Party.

“Keir Starmer, this is for Gaza,” Mr. Galloway said, wearing the fedora hat that has become his trademark. “You have paid, and you will pay, a high price for the role you have played in enabling, encouraging and covering for the catastrophe presently going on.”