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

Frequent Trump administration critic Sen. Jeff FlakeJeffrey (Jeff) Lane FlakeHow fast population growth made Arizona a swing state Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden 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 TrumpNew Biden campaign ad jabs at Trump's reported 0 income tax payments Ocasio-Cortez: Trump contributed less in taxes 'than waitresses and undocumented immigrants' Third judge orders Postal Service to halt delivery cuts 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 ManafortFBI official who worked with Mueller raised doubts about Russia investigation Our Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam 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 PapadopoulosTale of two FBI cases: Clinton got warned, Trump got investigated Trump says he would consider pardons for those implicated in Mueller investigation New FBI document confirms the Trump campaign was investigated without justification 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.