He's taken on President Donald Trump and the Federal Reserve. Now, libertarian former congressman Ron Paul is taking on bitcoin.

According to Paul, cryptocurrencies have become an asset that rivals the bubble he sees in stocks.

"I think it's going to continue to do exactly what it's doing. It's going higher and it's going lower," he said Tuesday on CNBC's "Futures Now." "We can look at what's happening now, which to me is a climactic end of QEs."

Paul, who has done commercials touting currency competition for a company that benefits from bitcoin's rise, views the crypto craze as a side effect of central banks doing several rounds of quantitative easing to cope with the last financial crisis.

"I look at the problems we face. I think they're gigantic and people are desperate and looking everywhere. Why would they buy bonds that pay negative interest rates? Why would they buy stocks, and say well this time it's different? " asked Paul. "Cryptocurrency is a reflection of the disaster of the monetary dollar system."

Paul, who's also a medical doctor and former Republican presidential candidate, argues that cryptocurrencies are in an "exponential bubble" where trying to calculate its real value is extremely difficult. Bitcoin, the largest of the cryptocurrencies, has been trading above $17,000.

He hasn't been able to pinpoint when a plunge could happen in cryptocurrencies or the stock market. But Paul says the danger is real.

"They're both big bubbles in the sense that it occurred because there was excessive credit. But if you look at the curves, I think that the cryptocurrency curve looks more threatening," Paul said.