Ken Paxton, the Trump-endorsed and MAGA-backed insurgent, ousted Senator John Cornyn in a runoff on Tuesday, becoming the second primary challenger to knock out an incumbent Republican senator in less than two weeks in a raw display of President Trump’s powerful hold on the party base. The contest was the most expensive primary in American history — and Mr. Paxton prevailed despite being outspent on advertising by pro-Cornyn forces by roughly $80 million. Now, Republicans are bracing for a potentially competitive general election in Texas, where Democrats have not won statewide in a generation. Democratic donors nationwide have swooned for...