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

Frequent Trump administration critic Sen. Jeff FlakeJeffrey (Jeff) Lane FlakeGOP strategist donates to Alabama Democrat Sasse: RNC help for Roy Moore 'doesn't make any sense' Sasse calls RNC decision to resume support for Moore 'bad' and 'sad' 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.

Mueller is investigating possible collusion between President TrumpDonald John TrumpHouse Democrat slams Donald Trump Jr. for ‘serious case of amnesia’ after testimony Skier Lindsey Vonn: I don’t want to represent Trump at Olympics Poll: 4 in 10 Republicans think senior Trump advisers had improper dealings with Russia 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 ManafortJudge warns Manafort not to discuss case with media Manafort involved in drafting op-ed defending his Ukrainian work: court papers Trump went off on Manafort for suggesting he should not appear on Sunday shows: report 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 PapadopoulosMueller team questions how much Trump knew on Russia contacts: report Papadopoulos lied to FBI out of loyalty to Trump: report White House was not aware Clovis testified before grand jury: report 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.