Rep. Tom Allen (D-Maine) announced Tuesday that he will challenge Sen. Susan CollinsSusan CollinsCollins: I'm not working with Freedom Caucus chairman on healthcare Mexico: Recent deportations 'a violation' of US immigration rules White House denies misleading public in aircraft carrier mix-up 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."