At an oil and gas conference in Pittsburgh Wednesday, one of Donald Trump’s top energy advisers warned that a Hillary Clinton presidency would harm the drilling industry.

Fracking billionaire Harold Hamm spoke to the Shale Insight Conference, telling the audience that he was impressed with Donald Trump when the two met while working on the Mitt Romney campaign and that he thought Trump was the right choice for the oil and gas industry.

Hamm is CEO of Continental Resources, one of the largest fracking firms in oil-rich North Dakota. The youngest of 13 children, he started his company in 1967 at age 21.

The company has flourished over the past eight years, as domestic oil production has nearly doubled during the fracking boom. That boom also lowered the price of natural gas, which has hurt the coal industry. But Hamm said regulations from the Obama administration are what have led to coal’s demise. He warned the crowd that Hillary Clinton would be an adversary for the oil and gas industry in the way he says Obama has been for coal.

“We don’t want to wait with oil and gas until we’re out of business to try to fight back. It’s too late then—you’ve got to get ahead of it. We have to step up now,” Hamm said.

Clinton promoted fracking as Secretary of State, but has also said she would not favor fracking in areas where local communities opposed it.

Hamm attacked the Obama administration’s decision to halt the construction of the Dakota Access pipeline after a federal judge denied a request by a Native American group to halt construction.

“What we’re seeing here is a complete and total disregard of what made America great. The one thing we always had working for our country was a very strong rule of law,” Hamm said.

Hamm also warned that a Democrat in the Oval Office would mean a Supreme Court that favors more regulation, which he said would be bad for the fracking industry.

“If that happens, you’re looking at 40 years of liberal rule. The impact of that on our industry is tremendous.”

Trump raised eyebrows in the oil and gas industry when he said he would support local and state bans on fracking. A few weeks later, Hamm said that Trump didn’t “understand” the issue when the candidate made the remarks.

Trump is expected to address the conference Thursday.