Nebraska's attorney general officially announced his candidacy for Sen. Ben Nelson's (D-Neb.) seat Wednesday.

Republican Attorney General Jon Bruning, who started an exploratory committee for a Senate run just after the Nov. 4 midterms, officially announced his candidacy for the Senate, challenging the centrist Democrat.

“I filed paperwork with FEC today making my candidacy for U.S. Senate official. We’re in! Formal announcement will follow in a few weeks,” Bruning wrote Wednesday on his Twitter account.

Nelson, one of the most conservative Democrats currently serving in the Senate, has been one of the GOP's main targets to beat in 2012. Bruning is the one of the earliest Republican challengers hoping to win Nelson's seat.

Nelson, nominated to the Senate in 2000, has made no official announcement on whether he will run again, but The Omaha-World Herald has said he is leaning toward another run.