It’s in the nature of campaign trail reporting to become tedious and devoid of news for long stretches of time, which in turn creates incentives for news outlets to sex up dispatches from campaign events with gauzy claims, like “Challenging Hillary Clinton, Bernie Sanders Gains Momentum in Iowa.”

The evidence for this assertion comes not from survey data, but from snapshots of campaign rallies in small towns, where Sanders—an insurgent candidate with a dedicated fan base—has been able to draw larger-than-normal crowds; and from anecdotes like this one about a local party chair jockeying for influence.

Kurt Meyer, the county party chairman who organized the event, sent a text message to Troy Price, the Iowa political director for Hillary Rodham Clinton. Mr. Price called back immediately. “Objects in your rearview mirror are closer than they appear,” Mr. Meyer said he had told Mr. Price about Mr. Sanders. “Mrs. Clinton had better get out here.”

These are colorful details, but they tell us very little about the state of the race in Iowa. That's probably because the story of Sanders' surge...

...is complicated by one crucial caveat.

This data is taken from Huffington Post’s polling aggregator, but other major aggregators tell a similar story. Sanders’s support among Iowa Democrats has increased substantially since he began campaigning, but not in a way that’s particularly meaningful if another candidate enjoys steady support from significantly more than half of that electorate. To be a threat to Hillary Clinton, Sanders needs to do more than outperform other dark horse candidates and potential dark horse candidates like Martin O’Malley and Vice President Joe Biden. As long as she’s the favored candidate of 60 percent of Iowa Democrats, then even a surging underdog will hit a brick wall at 40 percent. To win in Iowa, Sanders can't just build support, he needs Clinton's numbers to come down. That would take something big because Clinton's supporters aren't loosely committed. TPM’s Daniel Strauss puts the Sanders surge in perspective.