Representative Seth Moulton on Thursday called Russia the biggest threat to national security in the short, medium, and long terms and said the only way to combat that threat is if Democrats and Republicans in Washington cooperate.

Moulton, an Iraq veteran and member of the House Armed Services Committee, said before Donald Trump became president, he considered terrorism the biggest short-term threat, Russia a medium-term threat, and the rise of China the greatest concern in the long term.

Now, he said, Russia is the biggest concern in all three categories.

“It has almost become like a sport to make jokes about Trump and Russia and everything that’s going on,’’ Moulton said. “But the national security implications here are deadly serious.’’

Moulton, a Democrat, spoke in an hour-long interview with reporter Joshua Miller before a packed room in the AT&T store on Boylston Street, part of The Boston Globe’s Political Happy Hour series.

Moulton said it’s important to get to the bottom of Russia’s motives in meddling in US affairs and called for an independent, bipartisan, balanced investigation into allegations of collusion between Trump associates and Russian officials.

The Trump administration is being investigated by the FBI for colluding with the country’s biggest enemy in the last 65 years, he said.

“Watergate is child’s play compared to this,’’ he said.

But more concerning than the question of how Russia interfered in the 2016 is election is what it might do next time, he said.

Moulton added that the only serious way to stop such a threat is if Democrats and Republicans cooperate.

“This is really serious stuff, and it’s a huge threat that we need to be worried about,’’ he said.

Moulton, a second-term Democrat who represents communities in the Merrimack Valley and on the North Shore, also criticized the president’s push to reboot the coal industry and cut environmental regulations.

He said Trump’s attempt to restart a dead industry is the wrong way to help the working-class people who look to the president for hope of seeing old high-paying jobs return.

Instead, the country should be focused on how to redirect those workers toward the “new economy’’ of technology, he said.

“The only people who are going to benefit from Trump’s coal push is the coal bosses, not the coal miners,’’ he said.

Trump’s attacks on environmental legislation will also hurt the people who live near coal mines, he said.

During the interview, Moulton spoke about his four tours of Iraq as a Marine after he graduated from Harvard University in 2001.

He told the story of a colleague from Iowa who, without any fancy college degrees or corporate pedigree, was one of the best leaders he has met.

He also revealed some personal details: For example, he does CrossFit with other members of the House of Representatives every day he is in Washington.

“It is a bipartisan workout group,’’ Moulton said.

“When you’re facing off on the floor doing pushups, and you say ‘You know Leader McCarthy, I’d really like your help on this bill,’ sometimes that actually goes better than on the floor of the House with the TV cameras,’’ he said, referring to House majority leader Kevin McCarthy.

About health care, Moulton said he is open to the idea of a single-payer system but said he has concerns about that type of national system after hearing horror stories about long wait lists in Canada.

“What we definitely need to do is minimize the influx of insurance companies who are kind of just middle men,’’ Moulton said.

The congressman said there also needs to be legislation to encourage more medical students to become primary-care doctors.

Moulton was also asked about his own political ambition. He said he does not plan to run for president in 2020 and is not considering challenging Senator Edward Markey in a primary race that same year.

“I just got elected to Congress two years ago. I don’t think that running for president is the right thought right now,’’ he said.

Moulton said he has also been asked about running against Republican Governor Charlie Baker for governor. He said he isn’t planning on it — because he wants to stay in Washington to keep an eye on Trump.

“I have my disagreements with Governor Baker but I think Governor Baker is a good man and he’s doing a pretty good job of leading this state, and Governor Baker most importantly is not a threat to the United States of America,’’ he said.

Moulton also said he doesn’t have a problem with Baker’s decision to join a panel convened by Trump to address the opioid addiction epidemic.

“If his joining that task force can save a couple lives then that’s a good thing,’’ he said. “I don’t think you put politics ahead of that.’’

Laura Krantz can be reached at laura.krantz@globe.com. Follow her on Twitter @laurakrantz.