When obstruction of justice is glaringly obvious
Jeff Sessions is out as attorney general, replaced by a bootlicking Trump stooge named Matt Whitaker. Given that President Trump openly said on national television in 2017 that he fired then-FBI Director James Comey to try to stop the Russia investigation, it's a safe bet that acting Attorney General Matt Whitaker — like Robert Bork before him — will be the cat's paw Trump will use to halt or impede Special Counsel Robert Mueller's Russia probe.
This would be obstruction of justice.
Let's examine Whitaker's career up to now. Before serving as Sessions' chief of staff, he was a U.S. attorney in Iowa from 2004 to 2009. He mounted a campaign for Democrat Tom Harkin's Senate seat, and lost to Republican Joni Ernst. Afterwards, he joined the board of a company called World Patent Marketing. What did this company do? The Washington Post reports:
The company was shut down earlier this year amid an FTC probe that accused it of being a sham group that cheated inventors by falsely promising them help with marketing their ideas in exchange for exorbitant fees. "For the last three years, Defendants have operated an invention-promotion scam that has bilked thousands of consumers out of millions of dollars," the agency alleged in a recently unsealed court filing. "In truth and in fact, Defendants fail to fulfill almost every promise they make to consumers." [The Washington Post]
At one point in 2015 Whitaker sent a threatening email to a complainer boasting of his U.S attorney record, writing, "I am assuming you understand that there could be serious civil and criminal consequences for you." (However, he was not personally accused of wrongdoing by the FTC.)
Before going to the Trump administration, Whitaker had a cushy job at a conservative nonprofit funded by dark money. In 2017, he published an op-ed on CNN's website arguing that the Mueller probe "is going too far," because it was looking into Trump's personal finances. (Heaven forfend!) He also tweeted an article calling this probe a "lynch mob."
Finally, he is good friends with Sam Clovis, a witness in the Mueller investigation due to his communication with George Papadopoulos (who pleaded guilty to lying to the FBI). In an ethical government, that alone would probably make Whitaker recuse himself as Sessions did. But more fundamentally, any Trump selection for attorney general, acting or otherwise, should recuse himself because Trump is the subject of the investigation. The conflict of interest is inherent and inescapable.
In U.S. law, obstruction of justice includes "endeavors to influence, obstruct, or impede the due administration of justice." In more plain language: "A person obstructs justice when they have a specific intent to obstruct or interfere with a judicial proceeding." For a concrete example, John Mitchell, H.R. Haldeman, and John Erlichman were convicted of obstruction of justice for (among other things) paying bribes to the Watergate burglars in an attempt to keep them quiet.
Whitaker's argument that Mueller doesn't have authority to investigate Trump's opaque personal finances is ridiculous. Whether you think he should or not, it is indisputable that Mueller does have wide powers: He has been tasked with investigating "any links and/or coordination between the Russian government and any individuals associated with the campaign of President Donald Trump," and "any matters that arose or may arise directly from the investigation," and then prosecuting any federal crimes he deems "necessary and appropriate." (Incidentally, it seems pretty clear the probe could go for a decade straight and not run out of people to prosecute.)
President Trump has already almost certainly committed obstruction of justice by firing Comey (he admitted he did it to stop the investigation on TV!) but directly shutting down or meddling with the investigation itself could not possibly be anything but obstruction. It would be the subject of a criminal investigation using his political authority to slow or stop that investigation. That is obstruction of justice.
Though they will surely try, not even the Trump hacks at The Federalist could come up with a definition of impeding or obstructing "the administration of justice" that doesn't include that. House Democrats, and the nation as a whole, better be ready to face up to this reality.