John Yoo — a conservative lawyer and a George W. Bush administration alum who authored the so-called “torture memos” — warned in an Los Angeles Times op-ed Tuesday that Donald Trump’s promise to appoint conservative judges is not enough of a reason to support him. Yoo said the concerns raised about Trump’s foreign policy proposals outweighed his vows on judicial appointments.

“While he is shaking up the world, Trump will also nominate conservatives to the federal courts — or so he says. But no one should rely on his vague promises,” Yoo wrote. “He has already flip-flopped on numerous core issues, such as the minimum wage, tax rates and entitlement reform. Even when he announced his list of judges in May, Trump would not be pinned down.”

Yoo cited the comments Trump has made suggesting he doesn’t understand basic facts about the U.S. legal system. He argued that, even if Trump did name conservative nominees, he would face obstacles pushing them through the Senate.

“Trump’s outbursts won’t persuade the Senate to embrace more conservative nominees, where Reagan’s sunny optimism and George H.W. Bush’s patrician decency failed,” Yoo wrote.

Yoo is not the only conservative legal scholar to express wariness over Trump’s claim that Supreme Court appointments are enough of a reason to back him.

Ilya Shapiro, a senior fellow in constitutional studies at the Cato Institute, told Huffington Post that “there’s a lot of uncertainty.”

“How hard would Trump push to get a nominee confirmed? What would he do if his first choice were rejected? Would he make a ‘fabulous deal’ to trade judicial appointments for other priorities?” Shapiro said.

Shapiro’s concerns were echoed by Richard Epstein — a Hoover Institution Fellow and professor at both New York University School of Law and the University of Chicago Law School — who told HuffPo that supporting Trump on basis of judicial nominees rested “on the questionable assumption that a man of his mercurial temperament and intellectual ignorance will keep to his word.”