Over the last 15 years, Mayes Middleton has become a prolific GOP donor, spending millions of his oil and gas fortune on conservative candidates and causes. His largesse has helped fund the rise of the hardline House Freedom Caucus and quietly enabled challenges to Attorney General Ken Paxton. Most of all, though, his money has fueled his own political rise, from a little-known oil and gas heir to a state legislator and, now, front-runner in the…