When Newt Gingrich said in a recent debate that he was a man of “grandiose” ideas, Mitt Romney’s campaign pounced. It sent mocking Twitter messages with a hashtag, “#grandiosenewt”, encouraging voters to add their own examples of occasions when they felt Mr. Gingrich had been “grandiose.”

Within minutes, the hashtag was trending on Twitter. Reporters picked up on it, sending out their own Twitter posts and writing their own articles. The result: for at least one news cycle, the Romney campaign had stamped a virtual “grandiose” on Mr. Gingrich’s forehead.

If the 2008 presidential race embraced a 24/7 news cycle, four years later politicos are finding themselves in the middle of an election most starkly defined by Twitter, complete with 24-second news cycles and pithy bursts.

With 100 million active users, more than 10 times as many as in the 2008 election, Twitter has emerged as a critical tool for political campaigns, allowing them to reach voters, gather data and respond to charges immediately. But like most new media tools, it also carries danger for the campaigns. It can quickly define the political debate, whether candidates like it or not, and a single 140-character missive can turn into a nightmare.