Former President Trump warned Thursday that Republican Kathy Barnette, who is making an unexpectedly strong showing in polls, will lose the Senate race in Pennsylvania if she captures the GOP nomination. Ms. Barnette, a 50-year-old conservative commentator and veteran, is receiving more attention in the closing days of the Senate primary battle, stealing the spotlight from her more well-known, and well-funded, rivals: celebrity physician Mehmet Oz, who has Mr. Trump’s endorsement, and businessman David McCormick. “Kathy Barnette will never be able to win the General Election against the Radical Left Democrats,” Mr. Trump said in a statement. “Dr. Oz is...