Washington (CNN) For one Democratic senator, Donald Trump Jr.'s meeting with a Russian lawyer is "absolutely" the smoking gun when it comes to the investigation into possible collusion with Russian election meddling -- and has "violation of law written all over it."

Sen. Jeff Merkley, the junior senator from Oregon who sits on the Senate Foreign Relations Committee, told CNN on "Erin Burnett OutFront" that Trump Jr. should not have accepted the June 2016 meeting with a Russian lawyer.

According to emails released by Trump Jr. this week, the lawyer -- Natalia Veselnitskaya -- was described to Trump's son in an email as a "Russian government lawyer" with information that was part of the country's effort to help elect his father.

When Burnett asked Merkley if in his mind the meeting is the "smoking gun" in the Russia investigation, he responded "absolutely."

Merkley added that he is skeptical -- given the presence of Trump Jr., Trump's son-in-law and senior adviser Jared Kushner and former campaign chair Paul Manafort at the meeting -- that then-presidential candidate Donald Trump did not know about it.

"All through this, Trump has maintained that there was no contact, no coordination, no collaboration. Clearly you have the mission, you have his Trump team, you have the Russians in the same room, you have the email that lays out it's part of an ongoing program, there's no surprise on the part of the Trump team," Merkley said. "(It's) really very unlikely that with the campaign manager, the son and the son in law all there that Trump knew nothing about this."

"My son is a wonderful young man. He took a meeting with a Russian lawyer, not a government lawyer, but a Russian lawyer," he said, while standing alongside French President Emmanuel Macron at the ornate Élysée Palace on Thursday. "It was a short meeting. It was a meeting that went very, very quickly, very fast."

But on Thursday, Merkley told CNN that he believes the significance of the meeting is not in its contents, but the expectation going into it.

"Their expectation was this was somebody tied in to team Putin," he said.

Merkley also said that he believes this was not the first instance of some sort of communication with the Russians, because of the Trump team's lack of surprise that the meeting's pretense was part of an "ongoing" Russian effort to support Trump's candidacy during the election.

"(Their) defense is that we didn't actually get anything good out of this meeting," Merkley said. "That's not the question. The question is whether you were aware of an ongoing Russian effort -- which this email lays out -- whether you were meeting to obtain information, which they admit that they were. And so this is definitely the type of collaboration that other events made us think might exist, but we hadn't seen it laid out so crisply."

Merkley also added that he thinks Kushner's security clearance should be reviewed in the wake of the news.

"There's so much about this that's just not right," he said.