Sen. Bernie Sanders of Vermont Announces 2016 Presidential Run: 'I Am Running for President' The senator from Vermont is set to announce his candidacy on Thursday.

 -- Sen. Bernie Sanders, I-Vermont, is running for president in 2016, Sanders said in an interview with ABC News’ Chief White House Correspondent Jonathan Karl.

“I am running for president,” Sanders said Wednesday. “I intend to stand up and fight for working families all over the country.”

Sanders, who is the longest-serving independent in congressional history, said he will run for president as a Democrat.

Sanders will hold a formal kick-off event in May, most likely in his home state of Vermont.

Sanders' entry in the race gives Democrats at least one liberal alternative to former Secretary of State Hillary Clinton, who declared her candidacy earlier this month. Sanders will also be Clinton's first formal Democratic challenger.

Sanders is a self-described independent socialist and will enter the race as a long shot.

The announcement comes at a time of growing liberal unease over President Obama's agenda and amid angst that Clinton won't be pushed sufficiently leftward with Sen. Elizabeth Warren, D-Massachusetts, taking a pass on the race.

Sanders, 73, currently serves as the ranking member of the Senate Budget Committee and previously served as the chairman of the Senate Committee on Veterans' Affairs.

Sanders first announced his presidential run in an interview with The Associated Press.

Watch "Good Morning America” for more from ABC News Chief White House Correspondent Jonathan Karl’s interview with Sen. Bernie Sanders.