Bennet -- who has served only 17 months but is still branded with a scarlet I(ncumbent) -- has raised dramatically more money than Romanoff and is using it to blanket local airwaves priced out of his competitor's reach.
There's a Joe Sestak angle here, too. Romanoff was also offered an administration job by White House fixer Jim Messina, presumably so he wouldn't create another pesky primary. Despite that contretemps, despite Romanoff's polish and aggression and hunger to win what many in the state believe should have been his to begin with, Bennet was on a bit of a roll. Until today, that is. We'll see whether the Arkansas effect plays out West.
Full disclosure: The Atlantic's editor is Michael Bennet's brother.