So hears Taegen Goddard at Political Wire:

A source tells Political Wire that Colorado Gov. Bill Ritter (D) is ending his re-election campaign. A fundraiser scheduled for tonight was canceled and all campaign staff was sent home in the middle of the day.

At Swing State Project, James L sees this as potentially good news:

While Ritter is the incumbent, his poll numbers have been flaccid all year, and he hasn't done himself any favors with the Democratic rank-and-file over the past four years with his borderline-hostile relationship with organized labor in the state. This could give Democrats the opportunity to start fresh and tap Denver Mayor John Hickenlooper for the race.

My hunch is that Jon Corzine's loss in the New Jersey governor's race has prompted a lot of Democrats to re-evaluate their prospects. The high unemployment rate probably doesn't help either.