Appearing on "Hardball" with host Chris Matthews, the senior Democratic member of the House Oversight Committee said former acting-Attorney General Sally Yates' upcoming testimony may expose when and how much President Donald Trump knew about former National Security Adviser Mike Flynn's Russian dealings.

Tuesday afternoon, CNN announced that Yates has stated that she gave a “forceful warning” to the White House about Flynn that went unheeded. Flynn was later forced out after the Washington Post reported he privately discussed U.S. sanctions against Russia with that country's ambassador to the United States during the transition period.

Addressing Yates' upcoming testimony, MSNBC host Matthews called it "the ball game."

"She alerted White House Counsel Don McGahn about discrepancies between the administration's statements on Flynn's contact with the Russian ambassador and what really transpired," Matthews explained. "It turns out, according to McGahn, that Flynn was caught having substantive conversations with the Russian envoy, the ambassador to this country, about sanctions. That's the serious business of this whole -- this is the ball game. If you can show that he was dealing with him on the sanctions issue as part of their whatever in-cahoots deal."

"The question is, Chris, why would he have been talking to the Russians about the sanctions?" Cummings replied. "And what did the president know about that? I mean, keep in mind he fired him. That is the president fired him. But he fired him three weeks after he knew it, after the president knew it, so why is that?"

"I think every day, there's a new question that opens up, and then of course the president cozies up to Putin," Cummings added. "So we've got to figure it out. and i think, again, this is a fight for the soul of our democracy.

Watch the video below via MSNBC: