Democratic U.S. Sen. Mark Pryor is kicking off his bid for a third term this weekend with a Little Rock fundraiser. But the campaign to unseat the Arkansas lawmaker is already in full swing.

Though Pryor doesn't have an announced Republican opponent, he's already been targeted by conservative groups that have been running newspaper, television and radio ads aimed at unseating the lawmaker next year. Pryor says the attacks are forcing him to start his campaign earlier than he'd like to.

One conservative political action committee has called defeating Pryor its top priority next year out of 33 seats on the ballot. Another group has launched TV spots deriding him as too liberal. They see him as vulnerable after President Barack Obama lost the state in the November election.