Republican Ken Paxton's 28-point primary rout of longtime Texas Sen. John Cornyn — the widest primary defeat for a sitting U.S. senator in nearly 50 years — further cements President Trump's hold on the GOP — but may complicate the party's ability to protect its Senate majority in November. Why it matters: As election returns rolled in late Tuesday, top Republicans stewed over Trump's endorsement of Paxton — Texas' attorney general with a long history of legal and ethical troubles who they believe could be more vulnerable in November. For Trump, it was the latest in a string of takedowns of Republicans he'd deemed not loyal enough. They include state lawmakers from Indiana who resisted his redistricting push and Sen. Bill Cassidy (R-La.), who in 2021 voted to convict Trump on impeachment charges. And now Cornyn, who almost always votes with Trump but was viewed as not aggressive enough in pushing his agenda. Cornyn's 28-point loss is the widest primary defeat for a sitting U.S.…