Donald Trump is running riot in the GOP china shop and gleefully tearing the place up. Consider the strength of Trump’s position: If he wins South Carolina by a big margin, he goes into Nevada with momentum, and the latest poll there has him leading by 26 points and pushing 50 percent. If he enters Super Tuesday a week later having won three out of the past three states — and with Cruz diminished by a South Carolina loss and Rubio having won nowhere — he could easily win, say, 10 contests that day. It might still be possible to...