Conservative billionaire power broker Charles Koch said Monday he is not backing Donald Trump, claiming the presumptive Republican presidential nominee’s principles are “antithetical” to his.

In an interview at the Fortune Brainstorm Tech conference in Aspen, Colorado, Koch spoke about Trump and presumptive Democratic nominee Hillary Clinton, saying: “I see two people that as of this point we’re not supporting.”

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“If I had to vote for cancer or a heart attack why would I vote for either?” Koch said when asked if he would cast a ballot for either candidate.

The decision to shun Trump could be a huge blow to Trump and the Republican Party.

Charles Koch and his brother David, each with fortunes estimated at more than $40 billion, have been major benefactors of conservative politicians, fueling the Tea Party movement and efforts that limit the size of government.

On Trump, Koch said, “I’m sure he’s a fine fellow underneath but when you look at our guiding principles, his guiding principles are in many ways antithetical to ours.”

Specifically, Koch condemned as a “monstrosity” Trump’s plan for tariffs on certain imports, saying a similar effort led to the Great Depression of the 1930s.

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Koch also said his comments in a previous interview about potentially backing Clinton were misconstrued.

“What I said was it was possible (to back Clinton) if she totally changed everything she stood for,” he said.

“I have got to be more careful about what I say.”

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The Koch brothers, who give few interviews, are at the forefront of a network of hundreds of wealthy donors who share the objective of minimizing the role of the federal government, shrinking taxes, eliminating regulations and generally resisting the policies of Democratic President Barack Obama.

Their Koch Industries conglomerate, with interests in chemicals, oil refining and electronics, is the second largest private enterprise in the United States, according to Forbes, with $115 billion in revenue in 2013.