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

Frequent Trump administration critic Sen. Jeff FlakeJeffrey (Jeff) Lane FlakeBiden nominates former Sen. Tom Udall as New Zealand ambassador Biden to nominate Jane Hartley as UK ambassador: report The Hill's Morning Report - Presented by Goldman Sachs - Voting rights will be on '22, '24 ballots 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 TrumpNew Capitol Police chief to take over Friday Overnight Health Care: Biden officials says no change to masking guidance right now | Missouri Supreme Court rules in favor of Medicaid expansion | Mississippi's attorney general asks Supreme Court to overturn Roe v. Wade Michael Wolff and the art of monetizing gossip 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 ManafortFormer bank CEO convicted of bribery in scheme to land Trump admin job Trial begins for Chicago banker who exchanged loans with Manafort for Trump job Legal intrigue swirls over ex-Trump exec Weisselberg: Five key points 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 PapadopoulosTrump supporters show up to DC for election protest Trump pardons draw criticism for benefiting political allies Klobuchar: Trump 'trying to burn this country down on his way out' 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.