"To me, it was the weakest performance by an American president ever," the Democratic senator from Virginia said in an interview with CNBC's "Squawk Box." Warner added that he is afraid Trump may have "gotten taken advantage of" in his private session with Putin.
Warner is the ranking Democrat on the Senate Intelligence Committee, which has been conducting a probe into Russian interference in the 2016 election.
On Monday, Trump sparked backlash from both Democrats and Republicans after he sided with Putin rather than his own intelligence services on whether Russia had meddled in the campaign. After his press conference with Putin, Trump tweeted that he has "GREAT confidence in MY intelligence people," but it did little to stem criticism of his performance.
Sen. John McCain, R-Ariz., a frequent critic of the president, offered a blistering take on Trump's meeting with Putin, calling it "one of the most disgraceful performances by an American president in memory."
After the Trump-Putin joint appearance, House Speaker Paul Ryan said in a prepared statement, that "there is no question that Russia interfered in our election and continues attempts to undermine democracy here and around the world."
Warner characterized Trump's refusal to criticize Putin as throwing the entire U.S. intelligence community "under the bus."
"I worry that if the president was so weak in public with Putin, what really happened in that couple hours session in private," Warner said, adding he is "unsure" how Congress can get that answer. "We sure as heck need a briefing."
Trump will meet with members of Congress on Tuesday after many lawmakers criticized his performance at the summit, the White House said.
The White House did not immediately respond to CNBC's request for comment on Warner's comments.
The Democratic Party's biggest financiers are reportedly pushing Warner, who is known on Capitol Hill as a moderate, to run for president in 2020. He is also a member of the Senate Committee on Banking, Housing and Urban Affairs and the Senate Finance Committee.