I want to echo Ilya’s recommendation of Politico’s instant symposium on the James Comey firing; I particularly liked these remarks from liberal Berkeley law professor Dan Farber, no fan of President Trump’s; I pass them along with Prof. Farber’s permission:

It’s premature but understandable that some people are calling James Comey’s firing a constitutional crisis. Comey violated clear Justice Department norms by his conduct while investigating Hillary Clinton’s emails. He compounded the offense by making false statements to Congress about Huma Abedin’s allegedly massive forwarding of sensitive emails to her husband, Anthony Weiner. If another president had fired another FBI director for such conduct in a high-profile case, few questions would be raised.
But this is not just any president or any high-profile case. The real concern here is that Trump has abused the power of his office to protect his political associates from criminal investigation. It’s not paranoia to think that Comey’s conduct concerning the Clinton investigation is merely a pretext for firing him, given that Trump applauded much of that conduct at the time. But we also have no proof that the justifications were pretextual.
We won’t really know how to interpret the Comey firing until we know whom Trump nominates to replace him. If Trump nominates an independent, respected figure to replace Comey, well and good. If he nominates someone who is compromised by associations with Trump or who lacks credibility as an objective investigator, then it will be fair to start making comparisons to Richard Nixon. That would be the point to start worrying in earnest about a constitutional crisis.