WASHINGTON—President Trump and Democrats rarely find themselves in alignment. Yet both sides wanted the same outcome in Tuesday’s Texas Senate primary runoff election. Ken Paxton’s trouncing of incumbent Sen. John Cornyn in the Republican runoff represents Trump’s latest triumph in maintaining his grip on his MAGA base after he similarly ousted rivals in Indiana, Louisiana and Kentucky. But to the delight of Democrats, the president’s decision to make an 11th-hour endorsement of Paxton could put the Senate seat in play for James Talarico after decades of Democratic futility in the Lone Star State.