First, Texas will not secede from the U.S., no matter what the Texas Republican party platform says. As Kevin Drum points out, the Texas GOP regularly pushes out an utterly insane platform and yet it doesn't matter.
Second, but what if the state did secede? With 40 electoral votes, and reliably Republican EVs as well out of the picture, would Republicans ever be able to win the Presidency again? With 40 EVs out, the winning threshold would be 250, not 270. Democrats have 209 reliable EVs, without TX Republicans would have only 125. So Democrats would already have a 84 EV head start before all the swing states fell into one column or the other.
Okay maybe it wouldn't make a Republican president impossible. But it would be a lot harder. Plus, if TX successfully secedes, I bet some other red state would try to do it too, which would further hurt the Repubilicans' chances.
