Rep. Tom Allen (D-Maine) announced Tuesday that he will challenge Sen. Susan CollinsSusan Margaret CollinsMcConnell: 'Whoever gets to 60 wins' on immigration GOP senators turning Trump immigration framework into legislation Longtime Clinton confidant blames Comey for 2016 loss 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."