70% of Americans identify as Christians (source: Wikipedia), and 50 to 70% of Christians are leaning Republican (source: Pew Research) (depends on denomination, Mormons and Evangelicals higher, Catholics lower). So we have 1/3 to half of the population. Do you think it's not large enough group to have strong political voice? I think it is. I'm not sure what "far out of proportion" claim is based on.
Correction: turns out 70% was for white evangelicals (missed a note), so the number is surely lower than half and probably around 1/4. The point still stands - 1/4 of the population is a huge block.
