• Sen. Mark Warner: "A slap in the face to the intel community."
• Sen. Bernie Sanders called it "reckless and dangerous" and said "Protecting our national security is one of the most important tasks a president has, and Trump is failing at it."
• Reps. Elijah Cummings and John Conyers: "We do not know if these allegations are true or false, but if — IF — these allegations are true…Trump…may have sunk to a dangerous new low."
• Sen. Martin Heinrich: Trump displays a "disturbing pattern of recklessness" guarding classified information.
• Sen Jack Reed: Trump has a "recklessness with sensitive information" which "is deeply disturbing and clearly problematic…I don't believe the President intentionally meant to reveal highly secretive information to the Russians…Something must change at the White House...The President's inexperience is no excuse for being irresponsible."
• Sen. Dianne Feinstein exited the Senate subway, was surrounded by reporters, and remarked "Oh my goodness. What's happened?" She added, "It does take a period of time to understand" what is classified and what isn't "and it's awful easy to slip."
• Rep. Nancy Pelosi called the disclosure "dangerous" and said "Congress must be given a full briefing on the extent of the damage President Trump has done."
• Rep. Adam Schiff: "Deeply disturbing."
• Sen. Chris Coons: "That's really shocking, and there's obviously going to be a lot more work for us this week…It's a reminder that the president, I think somewhat recklessly, chose to welcome the foreign minister of Russia and the ambassador [from] Russia into an oval office meeting..." this is deeply troubling."
• Sen. Chris Murphy: "Another very disturbing trend of careless behavior by this administration…I don't know when it will be enough for Republicans to understand that we need to get to the bottom of the connection between the president … and the Russian government."
• Sen. Kamala Harris: "If true, this is a serious threat to national security."

Read how the GOP responded here, how the administration responded here and how Trump has responded to leaks in the past.