President Donald Trump’s most prominent GOP critics on Capitol Hill are days away from completing their Senate careers, raising the question of who — if anyone — will take their place as willing to publicly criticize a president who remains popular with nearly 9 in 10 Republican voters. Sens. Jeff Flake of Arizona and Bob Corker of Tennessee engaged in a war of words with the president on myriad issues over the past 18 months, generating headlines and fiery tweets from a president who generally insists on getting the last word. Those battles put them on the outs with many...