So far, that’s been working out pretty well for him. Sure, he forced his way into a primary runoff against John Cornyn, a four-term incumbent senator who received tens of millions of dollars of support, a lopsided financial advantage in the most expensive Senate primary on record. All that money didn’t close the deal, and it generally means trouble when an incumbent faces a runoff. Mr. Paxton might very well be the G.O.P. nominee to face the Democratic Senate candidate, James Talarico, a mild-mannered Presbyterian seminarian Texas Democrats chose over their own more trollish candidate, Representative Jasmine Crockett.