Editor's Note: (Julian Zelizer, a history and public affairs professor at Princeton University and a CNN political analyst, is the author of "The Fierce Urgency of Now: Lyndon Johnson, Congress, and the Battle for the Great Society." He's co-host of the "Politics & Polls" podcast. The opinions expressed in this commentary are his own.)
(CNN) The news that former FBI director James Comey wrote in a memo that President Donald Trump asked him, in February, to stop investigating national security adviser Mike Flynn has sent shockwaves through Washington.
It comes amid Trump's worst week in politics, and that's saying a lot. After many months of shocking and aweing the nation, the President is finally doing something that seemed impossible -- he is threatening to crack the Republican firewall on Capitol Hill that has done more than anything else to protect him.
In general, House and Senate Republicans have stood firm as Trump has issued controversial statements and provoked international tensions. They have given him the benefit of the doubt, hoping that even though the President's first 100 days lacked any significant legislative accomplishments, the second hundred might allow the GOP to finally make progress on their agenda after eight long years in the wilderness.
Despite concerns that the health care legislation that passed the House would prove to be damaging to Senate Republicans, Senate Majority Leader Mitch McConnell said he still had confidence he could put the pieces back together into a successful initiative.
But in the span of just seven days, everything has changed. President Trump stunned the nation when he fired FBI Director James Comey in the middle of the agency's investigation into his campaign, then admitted in a TV interview that he did it because of the "Russia thing."
Presidential advisors were left tripping all over themselves trying to justify what happened, each seeming to contradict the other. They had no time to catch their breath before The Washington Post published a bombshell story Monday night alleging that Trump had extemporaneously shared highly classified information about ISIS with visiting Russian officials. While it is legal for the president to do this, providing this kind of sensitive information to a non-ally is highly unusual and, most experts agree, dangerous.
Sounding exasperated, McConnell told reporters, "we could do with a little less drama from the White House on a lot of things so that we can focus on our agenda, which is deregulations, tax reform and repealing and replacing Obamacare."
Then came the Comey bombshell.
The triple whammy of Comey's firing, the intelligence sharing with Russia and now the Comey memo has shaken the GOP, generating immense, open questions about whether the President is able to handle two core presidential functions that Republicans have always prioritized -- law and order and national security.
Now, more Republicans like Tennessee Senator Bob Corker, who warned of a "downward spiral," are speaking up. Others are not springing to Trump's defense and remain conspicuously silent. None of them know exactly what to do.
With an eye on the 2018 elections, they realize how politically damaging Trump could be to their prospects, particularly as his approval ratings are now hovering at about 38%.
The controversies of the past week have also taken place at a critical moment for Republicans. Members of Congress have been eager to move forward on some kind of legislative agenda. They wanted to make progress on some kind of health care bill and they were particular eager to move forward on Trump's proposal for supply-side tax cuts.
They were hoping that this would finally be the moment when Trump made a "legislative pivot" and actually took the job of pushing bills through Congress as seriously as he did his tweets.
Some have even hoped that, just maybe, despite all the odds, the President would be able to produce some kind of dramatic diplomatic breakthrough during his visit to the Middle East.
None of this looks like it is going to happen. The momentum has moved in the exact opposite direction. The President is now politically paralyzed and will be on the defense as the White House tries to manage fallout. With the pressure over ongoing Russia investigations and talk of mental competence and impeachment moving to the front pages, the President's men and women will be devoting much of their time to damage control.
A day before the headlines about Trump's Oval Office meeting with Russian leaders emerged, The New York Times had already reported that Senate Republicans were planning to go it alone to move bills.
"It does seem like we have an upheaval, a crisis almost every day in Washington that changes the subject," complained Maine Senator Susan Collins.
At bottom, wrote the conservative Times columnist David Brooks after the Washington Post story broke, "Trump is an infantalist." Senator Lindsey Graham said "If it's accurate, it'd be troubling." Senator John McCain tweeted: "If true, deeply disturbing."
House Speaker Paul Ryan said through a spokesman: "We have no way to know what was said, but protecting our nation's secrets is paramount. The Speaker hopes for a full explanation of the facts from the administration."
For all the talk about populism and his maverick appeal, the heart and soul of Trump's survival has been the power of partisanship. During his election campaign, he depended on most Republicans to support his version of the party line regardless of what they thought of him. And Republicans came through.
Since taking the oath of office Trump has counted on congressional Republicans to protect him from investigation or political backlash based on their desire to make the most of what is possible with united government.
To be sure, it is not clear yet if Republicans will actually break ranks. So far its been mostly talk and tweets. But the pressure is clearly building, and the voices of conservative protest are growing louder.
If the outrageous actions of the past seven days break this partisan unity, then President Trump will be truly exposed. If legislators feel that their majority is at stake and their party's long-term standing is in jeopardy -- particularly because of a President many of them did not like to begin with -- then anything becomes possible.
And at that point, this President may learn just how imperial the legislative branch can be when it puts its mind to taking on the White House.
This commentary has been updated from a version published earlier.