Flake on chaos of Trump campaign: 'Was that campaign even capable of colluding with anybody?'

Frequent Trump administration critic Sen. Jeff FlakeJeffrey (Jeff) Lane FlakeArpaio considering running for former sheriff job after Trump pardon Overnight Energy: Warren edges past Sanders in poll of climate-focused voters | Carbon tax shows new signs of life | Greens fuming at Trump plans for development at Bears Ears monument Carbon tax shows new signs of life in Congress MORE (R-Ariz.) on Sunday said he's unsure if the Trump campaign was "capable of colluding with anybody."

"I mean, I look at that last campaign, and think 'was that campaign even capable of colluding with anybody?' It was so chaotic and whatnot," Flake said on ABC News's "This Week."

"But the problem is, as they always say, the cover-up and the sensitivity that the White House has to the special counsel and this investigation is troubling," he added.

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Mueller is investigating possible collusion between President TrumpDonald John TrumpO'Rourke: Trump driving global, U.S. economy into recession Manchin: Trump has 'golden opportunity' on gun reforms Objections to Trump's new immigration rule wildly exaggerated MORE's campaign and Russia. His probe has led to two indictments and two guilty pleas thus far.

Former Trump campaign chairman Paul ManafortPaul John ManafortTrial of ex-Obama White House counsel suddenly postponed Top Mueller probe prosecutor to join Georgetown Law as lecturer DOJ releases notes from official Bruce Ohr's Russia probe interviews MORE and his former associate, Richard Gates, pleaded "not guilty" to charges of tax fraud and money laundering, among other things. Former national security adviser Michael Flynn and former Trump campaign aide George PapadopoulosGeorge Demetrios PapadopoulosFlynn, Papadopoulos to speak at event preparing 'social media warriors' for 'digital civil war' Judge dismisses DNC lawsuit against Trump campaign, Russia over election interference Mueller hearings should lead Democrats to be shocked at abuse of justice system MORE pleaded guilty to lying to the FBI.

Some Republican lawmakers have in recent weeks alleged bias within Mueller's investigation after it was revealed he removed an FBI agent from his team who allegedly sent anti-Trump text messages.

Trump has said he is not considering firing Mueller, but has lashed out at the FBI in recent days.

Flake said Sunday he "doesn't get caught up" in discussions over possible impeachment, but said it would be a "big problem" if Trump fired Mueller or pardoned Flynn.

"If the president continues to try to undermine the legitimacy of that investigation and if Republicans continue to try to help with that, I think that puts us in peril, that we shouldn't participate in the undermining of our institutions that way," Flake said.