DALLAS—A faceoff that could define the future of the Senate has ended with a defeat for Sen. John Cornyn (R-Texas) at the hands of Texas Attorney General Ken Paxton, who benefited from a late endorsement by President Donald Trump. The Associated Press called the race for Paxton at 9 p.m. ET in the May 26 election that caps off the most expensive Senate primary in history. Cornyn’s ouster marks the latest Republican to be defeated by a Trump-backed primary challenger, following Rep. Thomas Massie (R-Ky.), Sen. Bill Cassidy (R-La.), and several Indiana state senators in recent weeks. With 83 percent of the vote counted, Paxton led Cornyn by more than 28 points—a blowout against one of the most powerful Republicans in the Senate....