Hoyer's evolution on the issue of gay marriage follows a trajectory increasingly common for Democratic lawmakers: He started believing that civil unions would offer sufficient protections to gay couples and now believes that full marriage benefits are needed.
But Hoyer's journey comes with a twist: Along the way, Hoyer learned that one of his three daughters is gay.
In his first extended comments since he announced in May that he supports same-sex marriage, Hoyer said in an interview that his 1996 position on the Defense of Marriage Act measure was "probably a mistake."
"I have, throughout my career, thought we should treat people equally," said Hoyer, 73. "Like the president, I evolved into thinking marriage had a broader meaning."
The Republican challenging Hoyer for his seat, state Del. Anthony O'Donnell, opposes same-sex marriage and said Hoyer's views are "not in sync with the people of Maryland's 5th Congressional District."
"There are quite a few folks, especially in the faith community, who are very disappointed in his effort," O'Donnell said.
He said he wants to debate Hoyer and hear more about why he changed his mind. "I think the citizens deserve to hear him justify his position," O'Donnell said.
Hoyer said he learned in 2003 that his daughter, Stefany Hoyer Hemmer, now 43, is gay. But it was only after President Barack Obama announced in May his support for same-sex marriage that Hoyer decided to follow suit, issuing a brief statement.
Politico reported at the time that Hoyer had been "the lone gay marriage holdout among top Democratic congressional leaders."
Hemmer, meanwhile, said her father's decision to take a public stand emboldened her. She had kept her sexual orientation private from the wider world, revealing it only to family and close friends. But in June she went public.
She also announced she would be active in the campaign to approve Question 6, a referendum on Maryland's law to legalize same sex marriage. "I thought that if I came out, I could help push the effort forward," Hemmer said.
Father and daughter discussed same-sex marriage in separate interviews.
Hoyer, a member of Congress since 1981 who previously served in the Maryland Senate, said public support for the right of gay people to marry has "proceeded more quickly than any other civil rights issue I've seen."
"I think what people have seen is these are friends and neighbors and sisters and brothers and people you have a great deal of respect for," he said.
The congressman says his Baptist faith has had little influence on his views about this subject. And he said neither did the views of his children — all of whom had needled him on the issue for years. "All of my daughters would say, 'What is the deal, Dad?' " he said.
The larger influence, he said, came from a colleague: Rep. Barney Frank, one of the first openly gay members of Congress. "In discussions with him, I determined how important legal recognition of that relationship, of marriage, was."
Hemmer was married to a man for three years in her 20s, and the couple had a daughter. But Hemmer said she'd known she was different since adolescence. And while she'd eventually talked to her late mother about her sexual orientation, she never had discussed it with her father. "Women don't talk to their dads about their sex life," she said.
But if Hemmer did not talk to her powerful father about her sexual orientation, she did consider its possible consequences for his career.
"I did worry from time to time about folks finding out," she said. "I would date people. If something went bad in that relationship, what is to stop them from making a phone call?"
His positions on gay issues have mattered to her. She said she was "disappointed" when her father voted for the Defense of Marriage Act. "I didn't argue with him about it because, in his mind, he thought it was a language issue. Civil unions. People just weren't talking about it," she said.
"It took him a while to really evolve and say to himself: Why does it matter?" she said.
Since then he's taken other votes — and even led legislative battles for gay rights — that have made her proud. Hoyer was instrumental in repealing the Clinton-era "Don't Ask, Don't Tell" policy in the armed forces.
"I told him that he did a good job," Hemmer said. "I've applauded him when he's made good decisions. I think he appreciates it. I don't think he does it because he is making his daughter happy."
Hemmer said it made her proud this summer when her father announced his support for Maryland's referendum.
Mindful of his position, she still called to ask if it would be OK for her to a public stand on Question 6.
The congressman recalled that she phoned him and said: "Dad, I'm thinking of announcing my status and my support. Will that cause you a problem?"
Hoyer's response: "Of course not," he recalled.
Now she's joined the board of Equality Maryland, and she has traveled across the state raising money and telling her story. "Here is something that I can do maybe for this referendum," she said.