'Reminiscent of Nazi Germany': One of the most influential GOP donors takes aim at Donald Trump's Muslim proposals

One of the most influential GOP donors took a big swing at Republican presidential frontrunner Donald Trump in an interview aired Sunday.

In the ABC interview, Koch Industries CEO Charles Koch was highly critical of Trump's proposals toward Muslims on the campaign trail.

The GOP frontrunner last year unveiled a proposal to bar most Muslim immigrants and tourists from entering the US. He also expressed openness for creating a database to track Muslims in the US, though he later appeared to back away from the idea.

"Obviously that's antithetical to our approach," he said when asked about the proposed Muslim ban.

He then went on to lambaste the suggestion of a database.

"But what was worse was this: 'We'll have them all register.' That's reminiscent of Nazi Germany. I mean, that's monstrous," Koch said.

Throughout the interview, Koch - who, along with his brother is a key financial source for various conservative causes - did not express much apparent enthusiasm about the current crop of Republican presidential candidates.

The billionaire donor explained that he and his brother decided not to publicly support a candidate in the Republican race partially because they believed that the candidates did not focus enough on concrete policy issues.

"We said, 'Here are the issues.' You've got to be like Ronald Reagan and compete on making the country better rather than tearing down your opponents. Right off the bat, they didn't do it," Koch said. "More of these personal attacks and pitting one person against the other, that's the message you're sending the country. That's the way you should - you're role models, and you're terrible role models."

Despite being frequently pilloried on the left, Koch even left open the door to tepidly backing former Democratic frontrunner Hillary Clinton in a general election. He said it's "possible" that Clinton could be a better option than a Republican candidate.

The CEO's comments come after the political network headed by he and his brother announced plans last year to spend around $900 million to boost right-leaning candidates and think tanks in the lead-up to the 2016 election.