When properly motivated, Chuck Grassley can crush skulls. Right now, the Iowan octogenarian is just playing around.

Sen. Grassley, R-Iowa, warned the White House on Tuesday that he will lead a revolt against Trump's EPA nominees if the president supports a reduction of the Renewable Fuels Standard, that federal mandate which determines the amount of ethanol and other biofuels blended into gasoline and diesel fuel. But if Grassley really wants to scare off Trump, he will go after his judicial nominees.

As chairman of the Senate Judiciary Committee, Grassley can open and close that pipeline anytime. He controls when nominees get hearings and which nominees make it to the Senate floor for a vote. A Republican holding judges hostage would be an administration nightmare.

Frustrated legislatively, Trump has devoted much of his focus to the judiciary. Every confirmed judge represents a small but a lasting win, victories that the conservative base craves. Standing next to Majority Leader Mitch McConnell in the Rose Garden earlier this week, Trump heralded his success and called it "an untold story."

"Nobody wants to talk about it," the enthusiastic president said. "But when you think about it, Mitch and I were saying that it has consequences 40 years out, depending on the age of the judge—but 40 years out." Nodding enthusiastically along, McConnell noted that the Senate is in "the personnel business."

After putting Neil Gorsuch on the Supreme Court, the White House has managed to get six federal judges confirmed—four on the U.S. Court of Appeals and two on U.S. District Courts. Another 60 nominations are now filtering through the Judiciary Committee. Grassley knows and they're vulnerable. And Grassley knows how much he loves his ethanol, the demand for which is endangered by an administration threat to dial back the amount mandated.

Any slowdown in confirmations would deal a major blow to the administration. Trump would lose an opportunity to remake the judiciary in the image of the Judicial Crisis Network—one of the last major things Republicans can still agree on. Without judges, Trump looks even more impotent.

While the White House hasn't admitted this possibility publicly, it's the chatter of the petroleum industry, which loses business to the ethanol mandate.

Grassley already fired a shot across EPA Administrator Scott Pruitt's bow in a public letter condemning the use of private planes which lumped Pruitt together with former Health and Human Services Secretary Tom Price. That's telling.

Price flew private for more than $400,000 while Pruitt only took four noncommercial flights. Price chartered planes on demand while Pruitt cleared each flight with the EPA Ethics Office. And most importantly, Price wanted to overhaul Obamacare while Pruitt wants to hack away at biofuel subsidies—a reform that would hit Iowa hardest.

Grassley met with Pruitt Tuesday on Capitol Hill to discuss the EPA's looming November deadline to renew the Renewable Fuels Standard. Though a detailed readout of the meeting wasn't made available, it's easy to guess the general tone: "Nice administrative agency you've got here. Be a shame if something happened to all the bureaucrats you need to run it."

Luckily for the administration, nobody really notices EPA bureaucrats. They do care about judicial nominees though and if Grassley decides to take off the kid gloves, if he gets serious with Trump, the Iowa senator can cause significant pain.

Philip Wegmann is a commentary writer for the Washington Examiner.