Who does billionaire conservative prefer as president? It's a surprise

Democratic presidential candidate Hillary Clinton during a campaign stop at the University of Bridgeport in Bridgeport. Photo / AP

Conservative political activist Charles Koch suggested in an interview with ABC News today that Democrat Hillary Clinton would be a better US president than the Republican contenders.

However, he stopped short of saying he would support the former Secretary of State if she ends up representing her party in a general election.

The billionaire, who with brother David has been active in Republican Party politics, criticised the tone of the GOP presidential primary campaign, citing it as the reason the brothers have not contributed to any campaigns, including efforts to derail Republican front-runner Donald Trump.

In the interview with chief White House correspondent Jonathan Karl, Charles Koch said Bill Clinton had done a better job than George W. Bush in controlling government growth while president.

"So is it possible another Clinton could be better than another Republican?" Karl asked.

"It's possible," Koch responded.

"You couldn't see yourself supporting Hillary Clinton, could you?" Karl pressed.

Koch responded: "Well, I - that - her - we would have to believe her actions would be quite different than her rhetoric, let me put it that way. 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 has not and probably will not back any Republican in the waning weeks of the primary campaign because of the divisive rhetoric.

"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,' " he said.

"And right off the bat, they didn't do it. 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."

He slammed Trump's call for a ban on Muslims entering the country as "antithetical to our approach, but what was worse was this 'We'll have them all register' [notion]. That's reminiscent of Nazi Germany. I mean that's monstrous, as I said at the time."

Koch was referring to comments Trump made, then backed away from, suggesting that he was open to the idea of a database to track Muslims in the United States.

He also had harsh criticism for Senator Ted Cruz's threat to "carpet-bomb" Isis (Islamic State). "Well, that's got to be hyperbole, but I mean that a candidate, whether they believe it or not, would think that appeals to the American people - this is frightening."

Koch said he and his brother, who in the past have spent millions backing conservative Republican candidates and causes, have stayed out of the primary fight, including efforts to stop Trump from getting the party's nomination.

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

He suggested that he was moving away from that kind of heavy spending in national politics because the return on his investments "has been disappointing".

Asked whether he would sit out the presidential election, Koch said: "Well, we'll see. I mean, when we get a nominee, then we'll explore that. And we don't want arm-waving. We want to know specifics."