Mark Cuban is quickly becoming one of Donald Trump’s most blunt and fervent critics.

The Shark Tank host and entrepreneur took to Twitter on Monday to blast the Republican nominee’s knowledge of central banking policy, using it as proof that markets will fall if the businessman is elected in November.

The Tweets were sent after Trump appeared on CNBC Monday, saying that Federal Reserve chair Janet Yellen is keeping the interest rates down because of President Obama. “I have no doubt that’s the reason they are being kept down,” Trump added.

Cuban disagreed:

Following Trump’s remarks, Minneapolis Federal Reserve president Neel Kashkari went on CNBC himself to refute the nominee’s claim, saying that the Fed’s decisions are rooted in economic data.

“Politics simply does not come up,” Kashkari said.

 

Cuban, who once suggested he’d like to be a vice presidential candidate for either Trump or Hillary Clinton, has used Twitter and other forms of social media in recent months to question the candidate’s mental state and business acumen.

Cuban has also predicted, alongside much of Wall Street, that a Trump presidency would cause markets to fall. Investors hate uncertainty, and the Shark Tank investor said Trump is the epitome of it.

These criticisms don’t fall on deaf ears. In June, Trump allegedly reached out to Cuban to find out why he had gotten so negative. But Cuban never responded.