GOP senators who are facing primary challenges in 2018 include Sens.Marco Rubio (Fla.), Mike Lee (Utah), Mike Enzi (Wyo.), Rob Portman (Ohio), Tim Scott (S.C.), Rob Wittman (Va.), Ted Cruz (Texas), Mike Lee and Kelly Ayotte (N.H.).All are considered to be strong contenders to challenge President Donald Trump in 2020.MORE (R-Ohio), who has been a…