Sen. Mark Warner (D-VA), vice chair of the Senate Intelligence Committee, joined the chorus of senior Democrats calling for President Donald Trump to back off from his summit with Russian President Vladimir Putin — or at the very least, not meet with him alone.

But Warner took it a step further than Senate Minority Leader Chuck Schumer (D-NY), suggesting to reporters on Friday that if Trump and Putin meet one-on-one, the Kremlin leader would likely take advantage of Trump’s unpreparedness.

“I’ve been concerned for some time that the President’s ad hoc style of going into meetings and winging it isn’t appropriate, particularly when you’re dealing with someone like Vladimir Putin, who’s been on the world stage for 20 years, a former KGB agent,” the Senate Intelligence Committee vice chair said. “He will come in with his facts, with maps, and I’m afraid that actually the President could be taken advantage of.”

Warner’s comments come on the heels of Deputy Attorney General Rod Rosenstein’s announcement that 12 Russian intelligence officials had been indicted on charges related to conspiring to interfere with the 2016 election.

Ahead of his meeting with NATO allies earlier this week, Trump bragged to reporters that his summit with Putin would likely be the “easiest” part of his overseas trip.