Del. David Englin (D-Arlington) works on his laptop during the Jan. 23 House session at the Capitol in Richmond. (Steve Helber/The Associated Press)

For a half-dozen years, Democrat David L. Englin has been an unabashed and outspoken liberal voice in Virginia’s House of Delegates, arguing — usually in futility — against the Republican majority.

On Tuesday, state legislators began assessing what the 100-member chamber would be like without him.

Englin, barely three months into his fourth term representing parts of Alexandria and Fairfax and Arlington counties, said he will not seek reelection next year after an extramarital affair. He said he may resign before his term expires in January 2014. And one Democrat has already expressed an interest in his seat.

Englin, 37, a strong supporter of gay and women’s rights, health care and economic equality, surprised supporters and colleagues with his announcement Monday night at an Alexandria Democratic Committee meeting.

Alexandria City Council member Rob Krupicka (D) said that he and other Democrats who know the couple were shocked and saddened. “I really wish them the best. No one likes to see people hurt,” he said. “I think that’s a lot of people’s reaction.”

It’s a steep fall for an up-and-coming legislator with a safe seat in one of the most liberal districts in the state, and who had been mentioned as a possible candidate for Congress.

Del. Scott A. Surovell (D-Mount Vernon) called Englin a “rising star” in the Democratic Party. “David has one of the clearest political compasses in our caucus,” he said. “He is a brutally effective politician for someone in the minority.”

In recent weeks, Englin had gained national attention after he was the first delegate to use the term “vaginal probe” on the House floor when debating a contentious bill that requires women to have an ultrasound before an abortion. He quickly became a frequent critic of the bill and appeared repeatedly on Rachel Maddow’s show on MSNBC. His stance earned him a ribald ribbing from Del. David B. Albo (R-Fairfax) on the House floor.

Many in Richmond had heard the rumors of an affair, which led to his separation from his wife, Shayna, a political consultant, and repeated absences in the final days of the annual legislative session and last week’s ongoing special session. He missed key votes on the state budget and transportation funding and pension reform measures, but in a chamber dominated by Republicans, his vote would not have made a difference.

The Air Force veteran and former Eagle Scout told several colleagues about the affair and separation and, on Monday, announced that he wanted to work on his marriage. He issued a statement from him and his wife but did not return a call for comment.

“Political life has affected me in ways I never expected,” Englin said. “In my public role, I have worked hard to serve my constituents and to be an effective, progressive voice in Richmond. But in my personal life, I let down my family and was unfaithful in my marriage.”

House Republicans declined to comment Tuesday, but several Democrats praised Englin for being a hardworking, idealistic candidate who brashly took on a field of better-known, party-backed candidates.

Englin successfully sponsored a bill to ban smoking in restaurants championed by former governor Timothy M. Kaine (D), and a constitutional amendment on this fall’s ballot to allow the legislature to move its session if it conflicts with a religious holiday.

But most of the time, Englin, a fixture on the House floor, opposed efforts by Republicans: the delay of a $620 million payment to the retirement system; the cutting of millions of dollars to make education jobs in the expensive Northern Virginia market competitive; and the closing of meetings of committees of the state’s government reform panel.

Englin has been outspoken in his criticism of Republican Gov. Robert F. McDonnell and Attorney General Ken Cuccinelli II (R).

“He has been a tireless advocate for his constituents and for the equality of each and every Virginian,” said Del. Mark D. Sickles (D-Fairfax), chairman of the House Democratic caucus.

Englin had been among the possible candidates to succeed House Minority Leader Ward L. Armstrong after he lost his reelection campaign. Instead, Englin backed Del. David J. Toscano (D-Charlottesville).

Sen. Adam P. Ebbin (D-Alexandria), who served in the House with Englin until this year, was often confused with Englin: The two men spoke out on the same issues, had somewhat similar names, had their Jewish faith in common and came from the same part of the state.

“David had repeatedly stood up — not just for issues of social justice but economic justice,” Ebbin said. “It’s going to be a great loss.”

With his seat safe, Englin was able to say — or do — the unpopular. He introduced a bill that would have created a group to figure out how much money the state could reap if it legalized marijuana and sold it in more than 300 Virginia liquor stores. He flew to France on all-expenses-paid trips as part of an aggressive lobbying effort by a company pushing lawmakers to lift a ban on uranium mining in the state.

Englin said in his statement that he would leave politics to focus on rebuilding his relationship with his wife and son.

The couple married in 1996. As newlyweds, Englin and his wife altered their surnames to fuse a new, common surname. He had been born David English. Her last name was Wolin. They both took the last name Englin as a symbol of their matrimonial bond.

Shayna Beth Englin founded Englin Consulting in 2006 and teaches communications and public relations at Georgetown University’s School of Continuing Studies, according to the school’s Web site.

Last year, Shayna Englin worked as a consultant for Jaime Areizaga-Soto’s unsuccessful primary campaign for state Senate against former Arlington County Board member Barbara A Favola (D). Her firm also worked with Equality California, Campaign for Tobacco Free Kids, Democratic GAIN and, of course, her husband’s campaigns.