Rev. Delman Coates, senior pastor of Mount Ennon Baptist Church, in Clinton, Md., speaks at a Sept. 21, 2012, a news conference at the National Press Club in Washington. Maryland Democratic gubernatorial hopeful Heather R. Mizeur plans to name Coates as her running mate on Wednesday, according to a person close to the campaign. (Cliff Owen/AP)

Maryland Democratic gubernatorial hopeful Heather R. Mizeur plans to name Delman Coates, a Prince George’s County minister and outspoken proponent of same-sex marriage, as her running mate on Wednesday, according to a person close to the campaign.

Coates, 40, is senior pastor of the 8,000-member Mount Ennon Baptist Church in Clinton, and he has been an advocate of several progressive causes, including expanded access to health care, voting rights and immigration reform.

Mizeur, a Montgomery County delegate, will name Coates as her 2014 lieutenant governor candidate at a campaign rally in Silver Spring on Wednesday night, according to the person familiar with the pick, who requested anonymity to more freely discuss the campaign’s plans.

Coates declined to comment on Tuesday, as did Steven Hershkowitz, a spokesman for the Mizeur campaign.

Mizeur faces a competitive primary in June against two better-known and better-funded Democrats, both of whom have previously announced their running mates.

Lt. Gov. Anthony G. Brown (D) tapped Howard County Executive Ken Ulman (D) for his ticket in June, while Attorney General Douglas F. Gansler named Del. Jolene Ivey (D-Prince George’s) as his lieutenant governor pick last month.

Coates, whose congregation has ballooned in size since he became senior pastor in 2004, will become the first non-politician to join the ticket of a major candidate seeking to succeed Gov. Martin O’Malley (D) in next year’s election.

Coates, however, became something of a celebrity in Maryland political circles in 2012, when he testified alongside O’Malley in favor of a bill to legalize same-sex marriage in the state. The pastor, whose position was at odds with many other African-American ministers, later appeared in television commercials during a referendum over the legislation later that year.

The issue was also a leading priority for Mizeur. If elected next year, she would become the first female governor of Maryland, as well as the first who is openly gay.

Mizeur and Coates have also worked together in opposition to expanded gambling, arguing that Maryland should put a greater priority on cultivating more cutting-edge industries.

With Mizeur’s selection of Coates, all three Democratic tickets will have a representative from Prince George’s, a majority African-American county that is home to more registered Democrats than any other jurisdiction in the state. Brown is a former Prince George’s delegate, and Ivey is the current chairwoman of the county’s delegation in Annapolis.

Coates is a graduate of Morehouse College, and he holds a master’s degree from Harvard Divinity School and a master’s and doctorate from Columbia University.

Coates and his wife, Yolanda, have two sons, ages 10 and 7, and 4-year-old twin girls.