By Steve Holland CHARLESTON, S.C. (Reuters) – Before Donald Trump's put-downs and the stumbles in presidential debates, and before the profound frustration of voters became so unmistakably apparent, Jeb Bush appeared to be best positioned to win back the White House for Republicans in 2016. Now with his polling in single digits, strategists say his presidential ambitions may rest in large measure on the outcome of Saturday's South Carolina Republican primary. To be sure Bush has shown signs of momentum in recent weeks, with stronger performances in debates where he struggled earlier in the campaign and a better finish than Senator Marco Rubio of Florida, his fiercest rival for the Republican establishment vote, in the New Hampshire primary.