Sen. Tim Johnson (D-S.D.) is set to announce Tuesday that he will not seek reelection in 2014, Post Politics has confirmed.

The senator will speak "regarding the 2014 election" at 3 p.m. Central Standard Time from the University of South Dakota in Vermillion, S.D., his alma mater.

Two people with knowledge of his decision said Johnson will not run again -- a decision that was widely expected. The news was first reported by Reuters. A Johnson adviser declined to confirm his retirement.