Michael Gerson, a columnist and former speechwriter for President George W. Bush, says he knows how the eventual Democratic nominee can defeat President Donald Trump in next year’s election.

Trump has many potential vulnerabilities, Gerson wrote in The Washington Post, but whoever runs against him next year will need to narrow it down:

“My personal recommendation: relentlessly turning the president’s claim of authenticity against him, until his defining public attributes become national jokes. Every part of Trump’s appeal is fraudulent. His lies are not the filigree; they are the foundation.”

The “most disturbing” part of Trump’s inauthenticity is his lack of patriotism, Gerson claimed.

“Trump supporters must believe that deep down he is a true patriot,” he wrote.

“But the public evidence for this is scarce.”

Gerson also detailed Trump’s ties to Russia, his love of despots and the “casual slanders” he uses against his own country while attacking American principles. One example: When Trump defended Turkish President Recep Tayyip Erdogan’s crackdown on dissent.

“When the world looks at how bad the United States is, and then we go and talk about civil liberties, I don’t think we’re a very good messenger,” Trump said at the time.

Gerson said that gives Democrats an opening.

“I never imagined that the party of Ronald Reagan would cede the ground of patriotism,” he wrote. “But in the Trump era, Democrats have a chance to seize it.”

Read the full column here.