SAN ANTONIO--More than one political venture has met an unseemly end in the hardscrabble landscape of South Texas. It was here, in 1948, that Box 13 gave Lyndon Johnson a dubious 87-vote primary victory, crushing former Gov. Coke Stevenson's Senate aspirations. It was here, two years ago, that a bitter primary fight for a House seat shattered a friendship between two Hispanic Democrats. And it was here, this month, in a rematch of that contest, that a blogger-led quest to defeat a mainstream Democrat and drive the party further to the left smashed head-on into the realities of local politics....