Let's walk through how this could play out against Trump.
1) Senate Republicans can't justify Trump's firing of Comey
Democrats are pretty sure Trump fired Comey to get rid of the guy who was investigating his campaign.
We're not so sure what Republicans think. And it matters whether Senate Republicans ultimately decide they can justify Comey's firing. There's no filibuster anymore for presidential nominees, because Democrats got rid of it in 2013. Republicans hold a majority in the Senate, which means they could conceivably approve Trump's nominee over every single objection from Senate Democrats.
But will they want to?
Some, like Sen. Richard Burr (N.C.), chairman of the Senate Intelligence Committee, aren't giving Trump the benefit of the doubt that he had reasons to fire Comey.
Others, like Sen. Lindsey O. Graham (S.C.), are giving the president the benefit of the doubt: Comey had made several mistakes with political implications (re: Hillary Clinton's emails), so maybe it's best to start over, they're saying.
Republicans only hold a slim 52-vote majority in the 100-member Senate, so Trump needs pretty much every Senate Republican to see things his way. Not helping: Trump's own staff is struggling to explain to reporters why Comey was fired now. White House press secretary Sean Spicer literally hid behind hedges at the White House on Tuesday night instead of briefing reporters.
2) Trump nominates someone who is overtly political
Comey was fired less than 24 hours ago, so it's not clear who that person would be. But even some of Washington's most partisan Republicans have urged the president to appoint someone “independent minded.”
There's a consensus in Congress that the FBI is supposed to be above politics, for the same reason there's a consensus in the United States that courts shouldn't be politicized: Justice is supposed to be blind, not Republican or Democratic.
Of course, justice is in the eye of the beholder. When Trump nominated Neil M. Gorsuch to the Supreme Court, Democrats recoiled at what they saw as a right-wing ideologue judge. Republicans cheered his supposed impartiality.
Gorsuch will surely decide very important cases for the nation, but because the FBI director was in the middle of untangling how Russia got its teeth in the U.S. presidential campaign, there could be more pressure from Republicans on Trump to appoint someone to the FBI who everyone can agree is independent.
If he doesn't, it could lead to our next step in this process.
3) Congress decides to appoint a special prosecutor or independent committee on Russia
A quick recap of where we're at: 1) not being able to justify Comey's firing + 2) not being able to justify Trump's desired replacement = 3) the decision to put a congressional check on the FBI's investigation.
As I explain in a rundown of the four other types of investigations besides the FBI's:
A handful of House and Senate congressional committees are knee-deep in their own investigations of Russian meddling and possible Trump collusion, but at least one has already been mired in politics. And the public has very little confidence that Congress can conduct these fairly. An April NBC News-Wall Street Journal poll found that 73 percent of Americans want an independent investigation.
Congress could set up an independent commission, like it did after the attacks of Sept. 11, 2001, to look at what went wrong. The commission cannot prosecute for crimes, but it's the most independent tool Washington has for investigations.
Congress (or the Trump administration) could also pick a special prosecutor — usually outside the confines of government — to investigate potential wrongdoing, and, if it finds it, prosecute. (Like the investigation that led to Bill Clinton's impeachment by the House.)
Both those would take an act of Congress. Despite public support for something more independent than the FBI or congressional investigation, there's no evidence that Republicans in Congress want to begin such an aggressive inquiry.
"Today we'll no doubt hear calls for a new investigation, which could only serve to impede the current work being done," said Senate Majority Leader Mitch McConnell (R-Ky.) on Wednesday morning.
If Trump does nothing to assuage Senate Republicans' concerns that his decision to fire Comey was political, it's easy to see how Republicans will conclude that they must take matters into their own hands to ensure the investigation's impartiality.