Kwame R. Brown’s resignation as D.C. Council chairman has his former colleagues looking to restore order by selecting a low-key, four-term veteran to succeed him on an interim basis.

Council members and staff indicated Thursday that support is coalescing behind Phil Mendelson (D-At Large), a quirky, studious lawmaker who has served on the council since 1999 and is known for championing progressive social issues. The remaining 12 council members are set to vote Wednesday on an acting chairman who will serve until a special election is held in the fall.

“We need a calm and steady hand, and that is Phil,” said David A. Catania (I-At Large). “He’s not the most dynamic, but given all the other sideshows we have going on right now, that is exactly what we need.”

Mendelson faces a strong challenge, however, from Vincent B. Orange (D-At Large), who lost a 2010 bid for the chairmanship to Brown (D). Mendelson and Orange jockeyed for support Thursday as the allegations against Brown expanded to include finance misdeeds dating to his 2008 campaign.

Orange, who served as Ward 5’s council member from 1999 to 2007 and highlighted irregularities in Brown’s campaign finances in 2010, suggested that Mendelson had not locked up the requisite votes and did not have the gravitas and political skills needed to lead a tainted council.

“I’m for keeping the council moving forward, maintaining the status quo while all of us as a collective body figure out the best way to address the upcoming elections,” said Orange, who roamed the John A. Wilson Building on Thursday, frantically trying to upend Mendelson’s apparent advantage.

The scramble for chairman, a position that holds great sway over the council’s legislative agenda, underscores how Brown’s departure has affected District politics and could change the face of city government. The council, and then voters, will select a new leader as several federal investigations continue into city government and campaigns, including Mayor Vincent C. Gray’s 2010 bid.

Brown stepped down Wednesday after being charged with including falsified documents on a home-equity loan application. Prosecutors said he used the proceeds to purchase a 38-foot boat, which he named “Bullet Proof.”

On Thursday, prosecutors filed an additional charge against Brown — aiding and abetting a campaign finance violation. The new charge is a local misdemeanor count, which is less serious than the federal bank-fraud felony he was charged with on Wednesday. But the new charge implies wrongdoing in his 2008 campaign for reelection as an at-large council member — the focus of federal scrutiny since a city audit last year identified more than $250,000 in unreported contributions and expenditures.

Brown is expected to plead guilty in separate hearings Friday in the District’s federal and Superior courts. He did not speak publicly Thursday, but as his former staff members worked inside his locked office in the Wilson Building, his former colleagues elsewhere in the building started jockeying over his successor.

‘We will overcome this’

Although Gray (D) and council members sought to reassure residents that the business of government continues, political and business leaders conceded that it could be months until D.C. affairs stabilize.

“While this is a bump in the road, and something we have to go through, and it doesn’t feel good, and we all feel lousy about it, we will overcome this,” said Barbara Lang, chief executive of the D.C. Chamber of Commerce. “We are picking up the pieces and moving on.”

The first step will be electing an interim chairman to replace Brown from among the council’s four at-large members. Chairman Pro Tempore Mary M. Cheh (D-Ward 3) has set a meeting for Wednesday to do so.

Mendelson and Orange are the only two at-large members who have shown interest in the job.

The next step will come in a special election later this year, when voters will choose a replacement to finish Brown’s term, which runs through January 2015. The Board of Elections and Ethics will meet before next Thursday to declare a vacancy in the chairman’s seat and schedule a special election to fill it. The election could be scheduled for Nov. 6, the same day as the general election.

The board could also schedule the election about month earlier — most likely Oct. 9 — but at the significant cost of holding early voting and opening the city’s 143 polling places a second time.

Attorneys for the elections board have determined that even if the special election is scheduled for Nov. 6, it would be considered a separate election and the same candidate could appear on both ballots. That would allow Orange, Jack Evans (D-Ward 2), Michael A. Brown (I-At Large), Muriel Bowser (D-Ward 4) and Marion Barry (D-Ward 8) — all up for reelection in November — to run for chairman without risking their current seats.

In either case, with city primaries having been held in April, the chairman contest stands to energize what was supposed to a sleepy summer of politics in the overwhelmingly Democratic District.

Mendelson, 59, said outright Thursday that he intends to run in the special election. Orange, 55, said he was “keeping my options open” about the race but expressed some concern that he might have to risk his at-large seat.

Evans is also not ruling out a bid. But Bowser, rumored to be interested in the mayor’s job, said she does not plan to run for chairman this year.

“I won’t be in that race,” Bow­ser said.

Insiders and outsiders

Numerous other candidates from inside and outside the government could also jump in the race, which will probably be fluid because of the potential for presidential-level turnout.

Kathleen Patterson, a former Ward 3 member who ran for chairman in 2006, said Thursday that she was “not a candidate,” although former supporters speculated that she could run a formidable campaign. Robert C. Bobb, a well-respected former city administrator who is a private consultant, is also seen as a potential contender. He did not return calls for comment.

In an interview, Mendelson said that he was optimistic about his chances and that he’s best suited to bring “stability” to the council.

Mendelson, who would be the second white chairman of the council since home rule, said he’s hopeful that his colleagues can coalesce around him as the “consensus candidate” ahead of Wednesday’s vote.

“We need an interim chairman who can settle things down and get us back on track,” the four-term lawmaker said. “I certainly know the council and care very much about the council as an institution and for the city. I think I have shown through my record over the past 14 years, I try to listen to voters in all the segments of the community.”

Mendelson, who has chaired the Committee on the Judiciary and lives in Takoma, got his start in District politics as a community activist in Northwest in the late 1970s. He later went on to work for then-D.C. Council Chairman David A. Clarke in the 1980s.

A self-described “nitpicker,” Mendelson has a reputation for being one of the body’s most socially progressive members, taking a leading role in the fight to legalize same-sex marriage in 2009 as well as being a reliable environmental champion.

But Mendelson has also been a staunch ally of Gray’s and could be politically hampered by the association should the federal investigation widen.

Gray, however, said he is optimistic that the District government will continue to operate.

“The city is moving right along, and we will continue to move right along,” Gray said. “I am convinced of that.”