Rep. Seth Moulton (D-Mass.) said Tuesday that if Donald Trump Jr.'s setting up a meeting with a Russian lawyer during the presidential campaign isn't treason, he isn't sure what is.

“If this isn't treasonous, I'm not sure what is,” Moulton tweeted.

If this isn't treasonous, I'm not sure what is. https://t.co/QrBaaf4psM — Seth Moulton (@sethmoulton) July 11, 2017

Moulton has been a frequent critic of the Trump administration, dismissing President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE's claims that the investigation into ties between his administration and Russia are a "witch hunt."

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His tweet echoed comments by Sen. Tim Kaine Timothy (Tim) Michael KaineBarrett seen as a front-runner for Trump Supreme Court pick Biden promises Democratic senators help in battleground states Second GOP senator to quarantine after exposure to coronavirus MORE (D-Va.) Tuesday that the investigation into Russian interference in the 2016 election is now going beyond charges of obstruction of justice and is “moving into perjury, false statements and even potentially treason.”

Trump Jr. released emails between himself and acquaintance Rob Goldstone earlier Tuesday about setting up a meeting with Russian lawyer Natalia Veselnitskaya, purportedly to gather compromising information about Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE.

"If it's what you say I love it," Trump Jr. wrote in one of his responses.

President Trump's eldest son has denied any wrongdoing in taking the meeting, but lawmakers have questioned his actions.

Sen. Susan Collins Susan Margaret CollinsJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally Momentum growing among Republicans for Supreme Court vote before Election Day MORE (R-Maine) on Monday said before the emails were released that Trump Jr. should speak with the Senate Intelligence Committee about the meeting.

Trump Jr. responded that he would be “happy” to talk with the committee.