State Auditor Hector Balderas filed his statement of candidacy with the Federal Election Commission last week and is expected to formally announce this week that he is running for Senate, according to a source close to him.
His entrance sets up a competitive Democratic primary with Rep. Martin Heinrich, who announced his candidacy earlier this month.
The Democratic Senatorial Campaign Committee recruited Heinrich to run, and Chairwoman Patty Murray (Wash.) recently offered public praise for the second-term Congressman, but she stopped short of endorsing him.
Democrats hoped to avoid a competitive primary in the race to replace retiring Sen. Jeff Bingaman (D). Republicans are expected to have a competitive primary as well, with former Rep. Heather Wilson already running and Lt. Gov. John Sanchez strongly considering a bid as well.