President Trump's abrupt firing of FBI Director James Comey was a grave moment in American governance that carries far-reaching risks.

Comey was sacked seemingly without warning and without a replacement lined up. The reason given for his dismissal — mishandling the investigation into Hillary Clinton's email server — is obviously false and so ludicrous as to be insulting. A flood of White House leaks this morning confirmed the near-universally held suspicion that Comey was fired because the president was unhappy with how the now-former FBI director was handling the Justice Department's investigation into the Trump campaign's ties to Russia.

So now we find ourselves in a situation in which a sitting president has offered a transparently bogus rationale for removing a high-ranking official charged with investigating potential misconduct by the president or his associates. The fact that Comey's dismissal coincided with other developments in the Trump-Russia investigation — like former acting Attorney General Sally Yates' testimony to Congress — makes the whole business still more suspicious and unjustifiable. The president, in his inimitably ham-fisted and self-defeating fashion, appears to be using his power to derail an investigation that is causing him political trouble.

This represents a monumental test for Republicans in Congress, and they will fail it.

Already we're seeing the critical Republican voices in the Senate — Majority Leader Mitch McConnell and Judiciary Committee Chairman Chuck Grassley — lining up behind the president's decision to fire Comey and dutifully parroting the White House's absurd talking points. A few scattered Republican legislators are voicing their concern, but none of that matters so long as the people in charge side with Trump. No one should delude themselves into thinking that a Republican Congress that obstructed a Supreme Court nominee for a full year because it feared losing political power will suddenly rise to the defense of governing norms.

This is a dangerous situation because the president clearly has no meaningful check on his excesses, and each time the Republicans in Congress make themselves accomplices to Trump's abuses will just embolden him to go further. If Trump were to nominate a sniveling loyalist to replace Comey (like, say, Rudy Giuliani) that person would stand an excellent chance of flying through to confirmation with the help of senators like John McCain and Marco Rubio, who will very publicly wring their hands while voting "yes."

But there is political danger for Trump and Republicans in all of this as well.

For Trump, the blundering abruptness of the Comey firing and the obvious conflicts of interest give Democrats a simple and digestible narrative to deploy against him: Trump fired Comey to derail the Russia investigation and then he lied about it. He's heaped another layer of controversy onto the Russia scandal that makes it more about Trump's own corruption than his campaign's connections to a foreign adversary.

For Republicans in Congress, it's another political grenade to jump on in order to protect their larger agenda, which has already absorbed a considerable amount of damage. We're already seeing Republican legislators beg off holding town hall meetings so that they can dodge the wrath of constituents who are already spitting mad about Russia, Trump's tax returns, the American Health Care Act, and various other items in the Trump scandal menagerie. Political analysts are beginning to talk (extremely cautiously) about the potential for a Trump-fueled blowout in the 2020 midterms that could return Democrats to power in the House.

If the Republican majority in either House comes under threat, then the danger to Trump grows greater still. The last thing the White House wants is for Democrats to gain any sort of investigative authority in Congress. As it stands, Trump can count on House Oversight Committee Chairman Jason Chaffetz to stymie any sort of real investigation into his administration's actions. That protection won't exist if Elijah Cummings takes over the gavel.

As for the GOP policy agenda, I don't think the Comey controversy will do much to alter its status. The reason the Republicans in Congress have so willingly signed on to Trump's destruction of governing norms is that they need him to pass tax reform and the health-care bill and all the other conservative legislative agenda items that have languished over the last decade. As it stands, they still have a chance to shepherd all those policies through, but that chance drops to zero the moment they turn on Trump.

So they'll stick with the president on the Comey firing and every other indefensible behavior right up to the moment that Trump ceases to be useful. After all, a tax cut signed by a corrupt president is still a tax cut, and a Federalist Society judge nominated by a wannabe strongman still serves a lifetime term.