A political novice who has three Harvard degrees and served four tours of duty in Iraq has decided not to run against Representative John F. Tierney, the Salem Democrat who is already facing a stiff challenge from a Republican candidate.

Seth Moulton, 33, who grew up in Marblehead, said Monday that he commissioned a poll that suggested that, if he ran, he would have been in “almost a dead heat” with Tierney and the Republican candidate, Richard R. Tisei.

But Moulton said he decided it would have been too difficult to organize a full-fledged campaign with only three months until the November election. He was also facing a July 31 deadline to collect the 2,000 signatures needed to qualify for the ballot.

“It all just comes back to the logistics,” Moulton said. “There just isn’t enough time.”

A self-described “fairly centrist guy,” Moulton said he would have run as independent and would have caucused with the Democrats if he had won.

His entrance into the race could have upended what has been a two-man battle between Tisei, the former Republican leader of the state Senate, and Tierney, who has been battling questions about the offshore gambling ring that was run by his brothers in law.

Moulton declined to discuss what reaction his potential candidacy had sparked among Democratic officials in Washington and Massachusetts. His candidacy could have siphoned off Democratic votes from Tierney, helping to elect Tisei, or it could have split the anti-Tierney vote, helping to reelect the incumbent congressman.

Moulton, who has never held elective office, said he had only begun exploring the idea of a run for Congress in the last several weeks.

He graduated from Harvard College in 2001 and says he decided to join the Marines in the summer of that year, before the September 11 attacks. He served two tours in Iraq as an infantry platoon commander and two tours as an assistant to General David H. Petraeus, in 2005 and 2007-2008. He then returned to Harvard where he received degrees from the Kennedy School of Government, and Harvard Business School.

He is currently managing director of Texas Central Railway, a company that is trying to bring Japan’s “bullet train” to Texas.

Moulton lives in Dallas, but says he moved there for the job eight months ago and has lived in Massachusetts for 32 of his 33 years.

On Monday, he said his long-term plan is to return to Massachusetts and run for office. “There is a yearning for new leadership in Washington,” he said, “and I saw that in the last couple of weeks.”