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

Frequent Trump administration critic Sen. Jeff FlakeJeffrey (Jeff) Lane FlakeSchumer recruiting top-notch candidate for McCain Senate seat The Hill's Morning Report — Trump eyes wall money options as shutdown hits 21 days Poll: Sanders most popular senator, Flake least 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 TrumpCoast Guard chief: 'Unacceptable' that service members must rely on food pantries, donations amid shutdown Dem lawmaker apologizes after saying it's never been legal in US to force people to work for free Grassley to hold drug pricing hearing 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 ManafortPavlich: Mueller’s indictment of the media Dem senator: 'Putin had something on' Trump which may account for 'plainly false' statements Debate builds over making Mueller report public 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 PapadopoulosHow the Clinton machine flooded the FBI with Trump-Russia dirt … until agents bit Barr: It would be a crime for president to pardon someone in exchange for their silence Press: What dirt does Putin have on Trump? 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.