The most retweeted presidential candidate of the night of the Republican debate wasn’t on stage Thursday. He wasn’t even in Cleveland. Or Republican.

Sen. Bernie Sanders (I., Vt.) was sitting with his campaign staff, tweeting his thoughts on the Republican debate — and his closing thought grabbed 19,000 retweets – more than any other White House hopeful. While the GOP candidates were onstage, their campaigns were actively tweeting, many using video clips from the debate. Still, none grabbed the almost 20,000 retweets that Mr. Sanders’ closing tweet saw.