WASHINGTON — Sen. Jeff Flake on Sunday called on Republicans in Congress to more forcefully push back against President Donald Trump and said there is growing concern behind the scenes that the president could take action against special counsel Robert Mueller.

“When the president says things that are just totally wrong, it’s the responsibility of members of Congress, particularly those in the president’s party, to stand up and say, ‘That is not right. Truth is not relative, and there are no alternative facts here,’” Flake, R-Ariz., said in an interview on “Meet the Press.” “And I have seen instances where we haven’t done that well, and we’ve got to do it better.”

Flake’s comments came in direct contrast to an assertion from Trump’s attorney Rudy Giuliani to The Washington Post this week that “truth is relative.”

And while Flake said Sunday there are some encouraging examples of bipartisanship when it comes to protecting the Justice Department’s investigation into the 2016 election, he believes his GOP colleagues have not gone far enough in speaking out against Trump’s criticisms of the probe. And, he said, Republicans are growing increasingly concerned privately that Trump could fire Mueller.

“I can tell you behind the scenes there is a lot of alarm,” Flake said. “There is concern that the president is laying the groundwork to move on Bob Mueller or [Deputy Attorney General Rod] Rosenstein, and if that were to happen, obviously, that would cause a constitutional crisis."

He added, "There is concern behind the scenes. I’ve been concerned that we haven’t spoken up loudly enough and told the president, ‘You simply can’t go there.’ And he’s obviously probing the edges as much as he can, to see how far Congress will go, and we’ve got to push back harder than we have.”

Trump has levied increasing attacks on the special counsel investigation through messages on Twitter, but Giuliani told CNN Sunday that there is “no discussion of firing Mueller.” Yet, Giuliani added that he felt the basis on which Mueller was appointed was illegitimate.

Giuliani — who also dismissed former Director of National Intelligence James Clapper and former CIA Director John Brennan as “two clowns” — added that the administration was comfortable with the investigation into whether there was collusion between the Trump administration and Russia, because “there has been none.”

“The obstruction part [of the investigation], I’m not as comfortable with, I’m not,” he said. “The president's fine with it. He’s innocent. I’m not comfortable because it’s a matter of interpretation, not just hard and fast true-not true."

Flake has been one of the president’s most vocal GOP critics in Congress. He announced last fall he would not seek another term, citing, in part, the challenges that come with winning a GOP primary while speaking out against the president. He acknowledged Sunday that part of the reluctance of his fellow Republicans to also speak out is out of fear that they may face a similar backlash from conservative voters.

“This is the president’s party, and if you’re running in a primary right now and you stand up to the president, or stand up in some cases for empirical truth, then you have trouble in primaries,” Flake said. “And that’s no doubt. So, I do think, as we get through the primary season, perhaps, then many of my colleagues will find a voice, but right now, it’s difficult politically.”

While he isn’t running for re-election to the Senate, Flake left open the possibility that he could challenge Trump in a primary in 2020.

While a presidential run is not in his plans, he said, he has “not ruled anything out.”