Republican superdonor Charles Koch shared scathing views on Sunday of the top two candidates in the party’s presidential nominating race, saying Donald Trump and Ted Cruz had presented policy proposals that were “reminiscent of Nazi Germany”, “monstrous” and “frightening”.

In an exclusive interview with ABC News , Koch, the libertarian billionaire head – with his brother, David – of vast energy and manufacturing interests, was asked about a Trump proposal to temporarily ban Muslims from entering the US. Koch replied that a corollary Trump suggestion , to create databases of Muslims, was worse.

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“What was worse was this: ‘We’ll have them all register.’ That’s reminiscent of Nazi Germany,” said Koch. “I mean, that’s monstrous.”

Koch held a slightly less dim view of a Cruz promise to carpet-bomb areas of the Middle East under Isis control. “Well, that’s gotta be hyperbole but, I mean, that a candidate, whether they believe it or not, would think that appeals to the American people?” Koch said. “This is frightening.”

The Koch brothers have spent untold millions over decades of efforts to reshape municipal and state governments, elect Republicans to congress, enact voter ID laws, combat gun control and discourage government regulation of all kinds. Early in the 2016 election cycle, David Koch predicted that Scott Walker, the governor of Wisconsin, would be the Republican nominee. The brothers backed Walker’s campaign, which did not survive 2015.

Koch on Sunday said that he and David were displeased with the tenor of the Republican race this year, to the extent that he suggested, perhaps facetiously, that they might even support the Democratic nominee-in-waiting, Hillary Clinton.

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“We would have to believe her actions would be quite different than her rhetoric,” Koch said. “But on some of the Republican candidates we would – before we could support them, we’d have to believe their actions will be quite different than the rhetoric we’ve heard so far.”

Koch said he could imagine Clinton as a preferable president over Trump or Cruz. “Let me put it that way,” he said. “It’s possible.”

The Koch brothers , who once vowed to spend nearly $1bn in the 2016 election cycle, were not actively spending money against Trump, he said.

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“We read– I read, oh, we’ve given millions to this one, millions to that one, and millions to oppose Trump,” he said. “We’ve done none of that. We haven’t put a penny in any of these campaigns, pro or con.”

He added though that he did not foresee a way of supporting Trump or Cruz. “These personal attacks and pitting one person against the other – that’s the message you’re sending the country,” he said. “You’re role models and you’re terrible role models. So how – I don’t know how we could support ’em.”

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