Democrats pounced on special counsel Robert S. Mueller III’s indictment of a dozen Russian military officers for their efforts to interfere in the 2016 U.S. election, with some saying Monday’s Donald Trump-Vladimir Putin summit should be canceled.

“These indictments are further proof of what everyone but the president seems to understand: President Putin is an adversary who interfered in our elections to help President Trump win,” Senate Minority Leader Charles E. Schumer said in a statement.

“President Trump should cancel his meeting with Vladimir Putin until Russia takes demonstrable and transparent steps to prove that they won’t interfere in future elections,” the New York Democrat said. “Glad-handing with Vladimir Putin on the heels of these indictments would be an insult to our democracy.”

Eighteen Democratic members of the House Foreign Affairs Committee quickly sent Trump a letter demanding the same following the indictment announcement.

“Russia has shown itself to be an adversary of the United States and our allies and a serious threat to global peace, security, and stability,” they wrote to Trump. “And, today, Special Counsel Mueller indicted twelve Russian military officers for hacking the Democratic National Committee — a direct attack on our democracy.”