Republican primary voters on Tuesday rejected U.S. Sen. John Cornyn, one of the Senate's most senior GOP members, in favor of Trump-backed Texas Attorney General Ken Paxton. Why it matters: Cornyn's loss marks one of the most significant defeats for a Republican senator in years and underscores how loyalty to President Trump continues to reshape GOP primaries nationwide. Trump gave Paxton a last-minute endorsement in the runoff. What they're saying: "Tonight is the beginning of the fight to preserve every value we hold dear," Paxton told supporters, referring to his upcoming general election campaign against Democrat James Talarico. "I've always supported the Republican ticket, and I intend to do so again in this general election," Cornyn told supporters in Austin. "I've said throughout this race that I trust the voters of Texas, and they made their decision." By the numbers: With 76% of the votes counted at 10pm, Paxton was up 28 percentage points over Cornyn.…