Cornyn is the seventh Republican Senator facing a primary challenge from the right . Republicans must defend 21 of the 22 Senate seats up for re - election in 2018 . Republicans say earlier tea party backed policies have hurt GOP .
Cornyn is the seventh Republican Senator facing a primary challenge from the right . Democrats must defend 21 of the 35 Senate seats up for re-election in 2014 . Strategists say earlier tea party backed challengers have hurt GOP .