JEFFERSON CITY, Mo. (AP) -- Republican attorney Ed Martin is getting into Missouri's U.S. Senate race.

Martin planned to make a formal announcement Monday about his challenge to Democratic Sen. Claire McCaskill. He is the second Republican to enter the primary, and several others are still considering it.

Martin narrowly lost a race last year against Democratic U.S. Rep. Russ Carnahan, of St. Louis. He previously served as chief of staff to former Missouri Gov. Matt Blunt.

McCaskill will be seeking a second six-year Senate term in 2012. Former Republican state Treasurer Sarah Steelman already has announced her candidacy for Senate.