Maryland Del. Heather Mizeur, running for governor, with her running mate, Delman Coates, in Silver Spring. (Karl Merton Ferron/Baltimore Sun)

Maryland Democratic gubernatorial hopeful Heather R. Mizeur on Wednesday named Delman Coates, a Prince George’s County minister and outspoken proponent of same-sex marriage, as her running mate in next year’s election.

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, formally announced Coates as her lieutenant governor candidate at a campaign rally Wednesday night in Silver Spring, calling him “a risk-taking change agent” and “a signal that we are done with politics as usual in Annapolis.”

“I am not just picking a running mate for an election season,” Mizeur told a boisterous crowd packed into an American Legion Post. “I wanted a friend, a confidant, a brilliant mind and a caring heart. . . . This is a man who knows how to move mountains by word and action.”

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

Heather Mizeur places her hand on her heart while talking to students at the Goodnow Community Center about the importance of being honest to oneself, in July. (Astrid Riecken/The Washington Post)

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

Among Mizeur’s biggest challenges is becoming better known across the state. Recent polls have shown that more than three-quarters of voters don’t recognize her name.

Coates, whose congregation has ballooned in size since he became senior pastor in 2004, is 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.

Mike Morrill, a veteran political consultant in Maryland, said Mizeur had made “an unusual choice.”

“The question voters will ask — and should ask — about an out-of-the-box choice like this is, “Is this person ready to step in and be governor if need be?” said Morrill, who is not working for any of the gubernatorial campaigns.

In an interview Wednesday, Coates said he saw the role of lieutenant governor as a way to build upon his ministry’s work.

“I’ve devoted my entire personal and professional life to improving the lives of people in our community,” Coates said. “This is really an extension of the servant leadership I’ve been committed to.”

He said he is joining a long tradition of religious figures who have moved into the political realm. Among those Coates named were Dwight Clinton Jones, the mayor of Richmond and a Baptist minister; and Walter E. Fauntroy, a Baptist pastor who served as the District’s delegate to Congress for nearly 20 years starting in 1971.

Coates 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, appeared in television commercials later that year before a referendum on the legislation.

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 worked together in opposing expanded gambling, arguing that Maryland should put a greater priority on cultivating more cutting-edge industries.

“I’ve known Heather for many years now and found her to be an ally on the issues we’re concerned about,” Coates said.

He said he has no reservations about becoming lieutenant governor without having held another elected office.

“While I have not been elected to public office, that does not mean I have not been serving the public,” Coates told the crowd Wednesday night, promising that the ticket would bring “transformational change” to Maryland.

With Mizeur’s selection of Coates, all three Democratic tickets now 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 chairwoman of the county’s delegation in Annapolis.

In a break with tradition, none of the tickets has a representative from Baltimore or Baltimore County. In the past, candidates from the D.C. suburbs have sought geographic balance by picking a running mate from the Baltimore region, and vice versa.

The closest in proximity is Ulman, and his county’s residents identify with both major regions in the state.

Coates is a graduate of Morehouse College, holds a master’s degree from Harvard Divinity School, and has a master’s and doctorate from Columbia University. He and his wife, Yolanda, have two sons, ages 10 and 7, and 4-year-old girls.