“Shameful.” “Disgraceful.” “A bad day for the U.S.” “The most serious mistake of his presidency.” That was how senior Republicans characterized the joint press conference Monday by President Trump and Russian President Vladimir Putin in Helsinki at the end of their summit.

The press conference appears to have accomplished the politically impossible. President Trump’s stunning endorsement of Putin’s denial of Russian meddling in the 2016 American election – contradicting the unanimous finding of 16 U.S. intelligence agencies – not only shocked Republicans, it spurred many of them for the first time in Trump’s presidency to join Democrats in criticizing him.

No one knows what the two leaders said – or may have agreed upon – in their two-hour, closed-door, one-on-one meeting. And yes, President Trump was right in asserting that there was nothing inherently wrong in meeting Putin. Efforts to improve U.S.-Russian cooperation on fighting terrorism and stopping the spread of nuclear weapons should be encouraged.

But President Trump’s statements Monday were not only highly problematic; they were a dangerous acquiescence to Putin that may have emboldened the Russian president and enhanced Russia’s place in the modern world. As such, the press conference – or the “surrender summit” as one critic called the Helsinki meeting – could be a defining moment in the Trump presidency.

At their joint press conference, President Trump made several critical errors.

First, there were the visuals. Putin seemed totally in control, whereas Trump lacked focus and authority, dodging questions and praising Putin when he should have been condemning him for Russia’s most egregious actions – the annexation of Crimea in 2014 and meddling in the last U.S. presidential election.

Second, rather than confront Putin about his election interference – as Trump had vowed twice to do, most recently in London – the U.S. president backed up the Russian leader point-after-point, especially on the meddling allegations.

When asked whether he believed Putin’s denial of interference in the election despite the U.S. intelligence community’s unanimous assessment to the contrary, President Trump referenced his defeat of Democratic presidential candidate Hillary Clinton and questioned why the FBI still did not have her server.

Even more stunning was President Trump’s assertion that he did not see “any reason why” Putin would interfere in America’s election, effectively siding with Russia’s leader over U.S. intelligence analysts.

National Intelligence Director Dan Coats quickly shot back, calling the U.S. intelligence finding “fact-based.” “We have been clear in our assessments of Russian meddling in the 2016 election and their ongoing, pervasive efforts to undermine our democracy,” Coats said, directly contradicting his commander-in chief.

Appearing ill-prepared and flummoxed by questions from reporters, President Trump fell back on his oft-repeated accusation that the FBI had failed to investigate thoroughly the rival he had defeated. At one point, the president even cited, erroneously, the Electoral College tallies from almost two years ago.

The American president seemed more interested in defending the legitimacy of his election than in protecting America’s national security interests.

Third, President Trump erred in asserting that the U.S. and Russia were both responsible for their troubled relations. He insinuated that America’s foreign policy missteps were the moral equivalent of Russia’s egregious conduct for over a decade, including: Russia’s invasion of Ukraine and annexation of Crimea; its shooting down of a civilian airliner; its aggressive behavior towards NATO allies Estonia, Latvia, and Lithuania in the Baltics; and its poisoning of a former Russian spy and murder of a British civilian in Britain by using a deadly nerve agent, Novichok.

Some of President Trump’s statements were simply baffling. Despite the U.S. and Russia backing different sides in the Syrian conflict, Trump suggested that he and Putin would soon begin working together to bring humanitarian aid to the Syrian people. This ignored the fact that Putin and his ally – Syrian dictator Bashar Assad – are largely to blame for the need for such humanitarian aid.

Equally mindboggling was President Trump’s agreement to consider Putin’s proposal that Russia and the U.S. work together to bolster cyberdefenses – the foreign policy equivalent of inviting the fox into the hen house to safeguard the eggs.

The response to Helsinki was instant and harsh. Democrats and Republicans alike complained that President Trump’s unwillingness to confront Russia had weakened the Western alliance and would encourage further Russian incursions into the territory of other sovereign nations. Putin now knows that Trump intends to give him a pass.

Most disturbing was President Trump’s unwillingness to support his intelligence agencies in their finding of Russian meddling. For a sitting U.S. president to say publicly that he believes a foreign leader more than his own intelligence team shocked many on both sides of the aisle.

Ailing Sen. John McCain, R-Ariz., – a frequent Trump critic – tweeted that the president had “abased himself” before a “Russian thug.” McCain said Trump had given “one of the most disgraceful performances by an American president in memory.”

“The damage inflicted by President Trump’s naiveté, egotism, false equivalence, and sympathy for autocrats is difficult to calculate,” McCain said.

Republican Sen. Ben Sasse of Nebraska called President Trump’s statements “bizarre and flat-out wrong.”

The president’s ravings were too much even for outgoing House Speaker Paul Ryan, R-Wis. “Russia is not our ally,” Ryan responded. “There is no moral equivalence between the United States and Russia, which remains hostile to our most basic values and ideals.”

Republican stalwart and former U.S. House Speaker Newt Gingrich called Trump’s performance “the most serious mistake of his presidency.”

Among Republicans, Trump had few defenders. Only his faithful Veep, Mike Pence, praised the summit performance, and Senator Rand Paul -- in a tweet.

Sen. Chuck Schumer, the New York Democrat and Senate minority leader, pleaded with Republicans to put aside partisanship and condemn what he called President Trump’s “insult to all Americans.” He urged Republicans to join Democrats in refusing to water down sanctions against Russia and asked them to call the chiefs of U.S. intelligence agencies before Congress to describe what they had told Trump before his meeting with Putin.

Schumer also urged President Trump to sit down with Special Counsel Robert Mueller so that Mueller could finish his inquiry into Russian election meddling. And Schumer urged Republicans to demand that President Trump insist that the 12 Russians intelligence officers indicted for hacking and other meddling on Friday be brought to the U.S. to stand trial.

Several times on Monday, Trump refused to make that demand. In an interview with Sean Hannity of Fox News, the president even expressed interest in Putin’s vague suggestion that Mueller’s team travel to Moscow to work with Russian security officials to investigate the Russian agents who were indicted in the U.S. for meddling.

How Republicans will respond to Democratic appeals for solidarity in the name of American national security is unclear. Thanks to the robust economy, the president’s popularity among Republican voters is strong and has been rising.

But at a time when our democracy faces grave threats, it is more than troubling that President Trump would side with the very country that has attacked us. Judging from the initial reaction, his performance has unsettled even those who usually fear him.

Ultimately, however, doing more than merely criticizing is vital to protecting American national interests. President Trump – and America, for that matter – can only win against Putin if we are assertive of American goals and our values. President Trump failed to accomplish that Monday.