Rep. Tom Allen (D-Maine) announced Tuesday that he will challenge Sen. Susan CollinsSusan Margaret CollinsLooking to the past to secure America's clean energy future Collins to endorse LePage in Maine governor comeback bid McConnell privately urged GOP senators to oppose debt ceiling hike MORE, who is considered one of the most vulnerable Republicans, in 2008.

"Maine and America face unprecedented challenges," Allen said in this video announcing his run. "For the last six years, the Bush Administration, supported by a Republican Congress, has failed the American people."