Arizona Republican Sen. Jeff Flake calls out Trump on trade

"The Constitution gives Congress the primary role in regulating trade, and certainly tariffs, not the President," Flake told CNN's Brianna Keilar. "So, the President is using an exemption that was never intended for that purpose."

Flake joined Senate Foreign Relations Chair Bob Corker, a Tennessee Republican, and a bipartisan group of senators Wednesday sponsoring legislation that would require lawmakers to approve movement on trade from the White House on the basis of national security, stymieing the administration's tariffs on Canadian steel.

"This is the first time that a number of senators have actually stood up, a number of Senators from the President's party and said, 'No, you can't do that, and we're going to pass legislation to rein this in,'" Flake said. "And so that is our prerogative, that's what we should be doing, that's what we should have been doing sooner than now."

Corker's bill has faced significant pushback --- CNN reported Trump called Corker Wednesday, encouraging the retiring senator to back off legislation in a "lengthy" call, while Republican Senate Majority Leader Mitch McConnell expressed his own antipathy, telling SiriusXM's Olivier Knox, "I don't think we need to be trying to rein in the President through legislation."

But Flake was optimistic Thursday, telling CNN, "We'll see. We have (the National Defense Authorization Act) bill coming up. Ever senator has significant leverage, because you have to have unanimous consent to move ahead to consider the bill," suggesting the action on trade could be attached as an amendment to the NDAA.

Flake's confidence seemed to rile the President, who tweeted shortly following Flake's interview, "How could Jeff Flake, who is setting record low polling numbers in Arizona and was therefore humiliatingly forced out of his own Senate seat without even a fight (and who doesn't have a clue), think about running for office, even a lower one, again? Let's face it, he's a Flake!"

Flake, for his part, pointed to the bill's bipartisan cosponsors, saying "I think there are about a dozen senators, some of whom are up for reelection this time or certainly will be in 2020 or who are standing up and saying, 'We are asserting our congressional prerogative.'"

When asked on Capitol Hill later for his response to Trump's tweet, Flake said he had "no reaction."

"I've said many times, this is a tough party right now for a Republican like me," Flake told CNN. "So, it is tough to get through a primary, if you agree with the President's positions or condone his behavior. So, I just can't do that."