By TIM BALK and REID J. EPSTEIN President Donald Trump said Wednesday that he would soon pick a candidate to endorse in the Texas Senate primary runoff between Sen. John Cornyn and state Attorney General Ken Paxton — and called on whomever he doesn’t pick to drop out. In a social media post, Trump did not specify when he’d announce an endorsement but stressed that Republicans must hold on to the seat in November. The winner off the runoff, set for May 26, will face James Talarico, a state...