A fiercely contested primary between Senator John Cornyn and Ken Paxton, the Texas attorney general, was upended in mid-May, when President Trump endorsed Mr. Paxton.

Donald Trump etched another notch in his kingmaker’s crown with Texas AG Ken Paxton headed to a showdown with James Talarico — a move Democrats believe could be his Senate majority’s undoing.

Experts say a Paxton victory would signal that Trump’s wing of the Republican Party has overtaken the Bush-era establishment, while a Cornyn win would suggest voters prioritize November electability.