Ron Paul, the libertarian who ran for president three times, said Tuesday he could possibly support Donald Trump "if he argued for the free market."

"But this is jokingly," Paul told CNBC's "Squawk Box." "Most people have rolled over and said, 'Yeah, we complained two months ago about tariffs, but he probably wasn't serious about it.' We don't know what he's serious about. Now, I think a tariff war would be horrible right now."

Trump, the presumptive GOP nominee, has said he would place a 45 percent tariff on Chinese goods in his attempts at making "America great again."