DETROIT — On the night before her Friday keynote address to a gathering of progressive activists here, Sen. Elizabeth Warren tried to slip into a hotel restaurant for a quiet dinner. But the former law professor has become a liberal superstar, and when a few admirers spotted her walking to the corner of the dining room, they cheered loudly. A moment later, more joined in the applause. Then one urged her, “Run for president!” The next morning at Netroots Nation, where Warren (D-Mass.) gave a fiery sermon for economic populism — “The game is rigged, and it isn’t right!” —...