Rep. Seth Moulton (D-Mass.) on Tuesday said that if members of the Trump administration or campaign have been "conspiring" with Russia, "that's the very definition of treason."

National Security Advisor Michael Flynn resigned on Monday after reports surfaced that he had spoken with a top Russian diplomat about the U.S. sanctions against Russia.

“I mean, let’s not lose perspective on exactly who we’re talking about here,” he said on CNN. "Russia is the No. 1 enemy of the United States of America.”

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“If members of the administration are essentially conspiring with Russia – either through the [2016 presidential] campaign earlier or now in the administration itself – I mean, look, Wolf, that’s the very definition of treason,” Moulton told host Wolf Blitzer.

“This is a very, very serious affair. The definition of treason is putting our enemy’s interests ahead of our own. That’s what the definition is,” he continued.

Moulton said Flynn's reported talk sanctions with Russia’s ambassador before Trump entered office is the latest example of the administration’s attempts at appeasing the Kremlin.

“It seems like there’s a lot of evidence that there are members of this administration that are more concerned with Russia’s goals than our own,” said Moulton, a member of the House Armed Services Committee.

“They continually prioritize the desires of the Kremlin, like, for example, rescinding these sanctions that have been so important."

Flynn resigned late Monday amid reports he misled senior White House officials, including Vice President Mike Pence Michael (Mike) Richard PenceGOP short of votes on Trump's controversial Fed pick Pence seeks to boost Daines in critical Montana Senate race The Hill's Campaign Report: Trump's rally risk | Biden ramps up legal team | Biden hits Trump over climate policy MORE, about his phone conversations with Russian ambassador Sergey Kislyak in December.

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The former Army lieutenant general blamed “the fast pace of events” for why he “inadvertently” gave Pence and others “incomplete information” about the talks.

Flynn, in an interview with the Daily Caller, said he spoke with the ambassador about the 35 Russian officials who were expelled from the U.S. as part of President Obama's sanctions against Russia, but "no lines were crossed."

White House press secretary Sean Spicer said Tuesday that Trump asked for Flynn’s resignation once the president’s faith in his adviser “eroded."

“This is not a legal issue but rather a trust issue,” he said, adding controversy over Flynn’s talks with Kislyak had created “an unsustainable situation."