Arkansas Lt. Gov. Bill Halter says he is seeking the Democratic nomination to challenge U.S. Sen. Blanche Lincoln this fall.

Halter's spokesman provided a statement Monday in which Halter said he would file papers for the Senate this week. Halter is the only Democrat to formally announce a challenge to Lincoln as she seeks a third term.

Lincoln faces a tough re-election fight, with eight Republicans already announcing interest in the seat. Groups on the left have criticized her stances on labor, health care and air pollution regulations.

Lincoln has become emblematic of the problems faced by Democrats, and more broadly, incumbents, in the 2010 campaign. She is considered by some political handicappers to be the Senate's most vulnerable member.

Halter is a former Clinton administration official. He was elected lieutenant governor in 2006.