Colorado Gov. Bill Ritter (D) is set to join the string of Democrats opting not to seek reelection this week.

Ritter's announcement is set for today, and he joins Sens. Byron Dorgan (D-N.D.) and Chris Dodd (D-Conn.) and Michigan Lt. Gov. John Cherry (D), who had been running for governor, in opting out of major races.

Democrats filled the hole in Connecticut quickly, with state Attorney General Richard Blumenthal (a notable step up from the embattled Dodd), but the other openings will be harder to manage.

Those mentioned as possible replacements for Ritter include: Denver Mayor John Hickenlooper, Interior Secretary Ken Salazar -- a former senator -- and former state House Speaker Andrew Romanoff, who is currently challenging appointed Sen. Michael Bennet (D-Colo.) in a primary.

Ritter served just one term and started out with a huge win over Rep. Bob Beauprez (R-Colo.) in 2006. Like many other governors, though, he has seen his numbers precipitously fall since then, particularly in the last year.

Rep. Scott McInnis (R-Colo.) is the likely GOP nominee.

Colorado is one of potentially two dozen competitive governors races this year.