What's with the surprise? Obama hit what was effectively the ceiling a Democrat can do in Texas in 2008 due to the unique circumstances, especially his and Clinton's lengthy primary campaign forcing them to build up campaign infrastructure in a state they otherwise never would have. Obama's 2012 result in Texas is him returning to the mean: his ~42% is basically what a Democrat is supposed to get in Texas. The Texas Democratic Party (an institution I'm actually very, very personally familiar with) is a corpse. It has no electioneering mechanism to speak of. Without Obama's own 2008 machine, there's nothing to see here.
Do you think someone could make Texas a swing state by giving the TDP an electioneering mechanism that works?