State Representative Charles A. Murphy’s surprise decision to resign this month has sparked sudden life into the race for his House seat, with at least three contenders in the fray.

Murphy, an eight-term Burlington Democrat, announced June 22 that he would not seek reelection and was vacating his seat effective July 31 to accept a job as vice president of public policy and government affairs for Burlington-based Arcadia Solutions, a health care consulting firm.

First elected in 1996, Murphy represents the 21st Middlesex District, which includes Bedford, Burlington, and a precinct in Wilmington.

Murphy had previously filed nomination papers to seek another term, and faced one challenger. Burlington Selectman Walter Zenkin, who is unenrolled, announced in May that he was running a write-in campaign in the Republican primary in September.

Since Murphy’s announcement, two others, both Burlington Democrats, have said they are running: David Fionda, a former town meeting representative, and Virginia E. Mooney, a longtime town meeting member and former selectwoman.

Anyone who decides to compete in the Sept. 6 primary will have to do so on a write-in basis.

“I’m really excited about it,” Fionda said. “I never thought this opportunity would have presented itself. I look forward to the next nine weeks. It’s going to be really interesting.”

Mooney, who has lost several previous bids for state representative and state Senate, said she looks forward to offering her candidacy again this year.

“I think I could bring a lot of common sense to the State House that’s missing,” she said.

Burlington Selectman Robert Hogan, who is also the town’s director of veterans’ services and a former chairman of the Democratic Town Committee, considered entering the race but decided against it.

An unusual dynamic of the race is that for the primary at least, the only name on the ballot will be that of the candidate who is not running. Murphy announced his plans after the June 1 deadline for withdrawing from the ballot.

Zenkin had originally intended to run as an unenrolled in November. But he failed to make the ballot due to a glitch in his nomination papers. Lacking sufficient time to circulate new papers before the filing deadline, he opted to pursue the write-in bid in the GOP primary, according to Burlington town clerk Amy Warfield.

With the filing deadline long past, Fionda, Mooney and anyone else who decides to compete in the Sept. 6 primary will have to do so on a write-in basis. To earn the nomination, a write-in primary candidate must collect at least 150 votes and finish first.

In the event Murphy collects the most votes in the Democratic primary and declines the nomination, the Democratic town committees in the district’s three towns would have to convene a caucus to choose a nominee.

Murphy, an ex-Marine, served as chairman of the House Ways and Means Committee for two years before he was bounced from the position by House Speaker Robert A. DeLeo in a January 2011 leadership shuffle. Since then he has been House majority whip.

“I’m thrilled; this is a positive move for me,” Murphy said in a Globe interview on the day he was leaving the House to take a new job. He called the decision “a little bittersweet,” though.

“I’m not going to focus on anything negative on my way out,” Murphy said. “It is what it is. The past is the past. You play the hand you’re dealt, and you move on.”

Fionda said he was shocked when he learned that Murphy, a friend whose daughter he has coached in youth basketball, had decided to resign. But it did not take him long to decide to jump into the race.

Within days of announcing on June 26, “I was able to set up my campaign organization, sign up my treasurer, get my Facebook page up and running . . . and start up a website,” he said. “So we’ve had a pretty busy and active week.”

In addition to having been an elected town meeting member for about six years, Fionda has been an appointed member of several town boards, including the Land Use Committee, and the Informational Services Advisory Committee.

Fionda has owned several business consulting firms, including a current one that helps professional service firms expand. He formerly ran a certified public accounting business.

“I think I can bring a fresh perspective. Having CPA skills and entrepreneurial skills and technology skills, bringing them to Beacon Hill would really benefit the voters of the district,” he said, adding, “I have a great passion to help others.”

A selectwoman from 1988-91, Mooney had previously lost bids for the board every year from 1974-87, and she has run for the board unsuccessfully every year since losing her seat. She has served as a town meeting representative for all but about four of the last 40 years.

Mooney, who is retired, said that in addition to common sense, she could offer voters “honesty and integrity,” qualities she said “we are losing every year.”

Zenkin could not be reached for comment.

Neither Fionda nor Mooney said they are daunted by the challenge of running on a write-in basis.

“Because of my background as an entrepreneur, I’m confident of my ability to outwork anybody, and to do whatever it takes,” Fionda said. “It’s a huge effort but I have a great committee.”

“It’s probably going to be easier than running a regular campaign,” Mooney said. “I just have to get my name out. . . . Before, I had to go out and have campaign stickers and signs. I don’t think I’ll need that with this kind of campaign.”