You could almost get the impression, reading Twitter, that so far the Republicans have been more interested in the Democratic presidential debates than the Democrats. Among Democrats, chatter about the early debates in Miami, Detroit and Houston has faded quickly under the daily gusher of breaking news. The race has tightened considerably, but the debates don’t yet seem to have produced a moment that has captured the Democratic audience. For Republicans, on the other hand, each large gathering of Democrats has been like a dopey, reassuring sitcom.