“We are literally living in the 10th anniversary of the month in which I knew I would be leaving the GOP,” D.C. Councilmember David Catania said on a February afternoon in his office on the fourth floor of the John A. Wilson Building.

The city’s first openly gay elected official remembers the evening, in 2004, that he heard President George W. Bush endorse a constitutional amendment banning same-sex marriage. Catania turned to his partner in the couple’s family room and said that was it when it came to his party affiliation.

“At that moment, I knew I would leave, but I was going to try to stay in to make as much of a difference as I could,” the 46-year-old recalled. “I knew I would be out before the year, but I wanted to keep the label because that label was a pain in their side. I was an elected Republican who happened to be gay and that visibility — that in and of itself — was a real problem for the Republicans.” Catania endorsed Sen. John Kerry, D-Mass., for president in 2004, and officially left the GOP in late September to become an independent.

Now, after four full terms on the council, he’s angling to become the first mayor of Washington, D.C., not affiliated with the Democratic Party. Catania will officially enter the race later this week, according to campaign aide Ben Young, arriving on the scene about three weeks before the April 1 primary that will set the ballot for November.

The combative, intense chairman of the council’s Committee on Education could face incumbent Mayor Vincent Gray, whose reputation took another hit Monday, after federal prosecutors alleged he had personally solicited funds for a shadow campaign that undermined his 2010 primary opponent, then-Mayor Adrian Fenty. But Catania could also find himself facing off with one of the four Democratic councilmembers jostling to take Gray’s place. He has been mulling a run for months, but word leaked out Monday from his campaign that he would make it official, hours after prosecutors detailed their case against one of Gray’s major backers, “Uncle Earl” Jeffrey E. Thompson, who pleaded guilty to providing generously to Gray’s shadow campaign.

Catania would have just more than six months to make his case for uprooting a 40-year tradition of Democratic rule. To some, he appears well-armed for the uphill fight.

A source with decades of experience on the D.C. political scene compared Catania’s smarts, sharp intellect and “suffer no fools attitude” to that of former Rep. Barney Frank, D-Mass., who is also openly gay.

Bolstering Catania’s case for the District’s top political job is the name recognition that comes with being a citywide elected official with a long record of leadership on high-profile issues that are popular with progressive voters, including the legislation to make the city smoke-free, establish gay marriage and legalize medical marijuana.

Plus, it’s not his first time playing the underdog.

Catania stunned D.C.’s political establishment in 1997 when he defeated prominent Democrat Arrington Dixon in a special election for an at-large council seat. Dixon had “crushing name recognition,” compared to Catania, then a 29-year-old lawyer, who was in his first year as an advisory neighborhood commissioner.

“As far as contradictions, you could stack them up,” Catania said. “You’re young, you’re new, you’re unknown, you’re a Republican, you’re gay — forget it.”

Catania easily won re-election in 1998, 2002, 2006 and 2010. Before taking the helm of the Education Committee, he helped expand access to pediatric care as chairman of the Health Committee.

It was in that capacity that he first met Cindy Rosenwald, a Democratic New Hampshire state House member. He convinced Rosenwald to take on the cause of voting representation in Congress for the District. Catania has traveled to New Hampshire to testify in support of Rosenwald’s resolutions three times — most recently in late February.

“I think he’s really smart and I’ve noticed that he really cares about the residents of the District a lot,” Rosenwald told CQ Roll Call. “He seems to have a comprehensive understanding of the diverse neighborhoods. … He’s also very brave.”

The resolution has been recommended for passage and could come up for a vote on the New Hampshire House floor as early as Thursday, Rosenwald predicted.

Catania calls New Hampshire his “first stop” in an East Coast tour aimed at building support for expanding D.C.’s rights in Congress. Next up is Vermont, to lobby his good friend Gov. Peter Shumlin, who is the current chairman of the Democratic Governors Association. They worked together on pharmaceutical legislation when Catania was leading the National Legislative Association on Prescription Drug Prices.

“I know he’s a great friend of the District,” Catania said, explaining his plan to win New England’s endorsement of D.C. statehood. “You’ve got to break that seal, then the others come rapidly.”

If elected, the Midwestern native promises to take his fiercely independent mindset to Congress. He’s got a “beef” with the idea of “internalized subjugation” to Capitol Hill and other institutions.

Catania said he never learned that where he grew up. He spent much of his childhood in the small Kansas town of Osawatomie, along the plains where abolitionist John Brown led his uprising.

“There is an ongoing competition as to who will govern us,” said the soon-to-be mayoral candidate. “Will it be the opinion formers of newspapers, members of Congress or will it be the elected leadership of the city?”