In 2012, the congressional scholars Thomas Mann and Norm Ornstein wrote a column for the Washington Post diagnosing what they saw to be the central problem in modern American politics.

"The GOP has become an insurgent outlier in American politics," they wrote. "It is ideologically extreme; scornful of compromise; unmoved by conventional understanding of facts, evidence and science; and dismissive of the legitimacy of its political opposition.

"When one party moves this far from the mainstream, it makes it nearly impossible for the political system to deal constructively with the country’s challenges."

The op-ed hit like a bomb. Mann and Ornstein were institutionalists with wide respect in both parties — Ornstein, in fact, worked (and still works) for the conservative American Enterprise Institute. For them to call out one party as "the core of the problem" in American governance was to violate all the rules of polite Washington society. Their diagnosis was controversial at the time, to put it lightly.

It is obviously correct now.

This week, it became clear that the Democratic Party will nominate Hillary Clinton — a politician about as mainstream in her beliefs and methods as you will find in American politics. It also became clear that the Republican Party is overwhelmingly likely to nominate Donald Trump — a man who is, by any measure, "ideologically extreme; scornful of compromise; unmoved by conventional understanding of facts, evidence and science; and dismissive of the legitimacy of [his] political opposition."

To put it differently, the Democratic Party, for better or worse, is practicing politics as usual. The Republican Party is embracing what David Brooks calls "antipolitics": leaders with "no political skills or experience" who are "willing to trample the customs and rules that give legitimacy to legislative decision-making if it helps them gain power."

If this were just about Trump, it could be dismissed as an aberration. But it's not just about Trump.

This week, President Obama nominated Merrick Garland to fill the Supreme Court seat left open by Justice Antonin Scalia's death. Garland leans left, but he's a clear compromise choice: older, more moderate, and with a long history of support from Republican senators.

As of now, Senate Republicans are refusing to give Garland — or any other Supreme Court nominee — so much as a hearing. Their position is, flatly, that they will refuse to confirm any nominee, no matter how qualified or appealing, until the next president is inaugurated.

In practice, what this means is they are hoping to hold the Supreme Court vacancy so it can be filled by ... President Donald Trump. They are refusing to do their institutional duty so that the decision can be made instead by a committed anti-institutionalist.

There is a deep pull in political punditry toward asserting symmetry between the two political parties — whatever sins one party is guilty of, surely the other party is no better. But this was a week in which the pretense of symmetry between the modern Democratic and Republican parties fell away.

The Democratic Party is acting like the political parties we have traditionally known in American politics: It is backing familiar politicians with deep institutional ties and, amidst divided government, nominating compromise figures with the potential for bipartisan appeal.

The Republican Party, however, is moving in a different and worrying direction: It is nominating an inexperienced demagogue whose appeal is precisely that he has no institutional ties, and it is refusing to even consider compromise with the sitting president.

In their 2012 column, Ornstein and Mann wrote, "When one party moves this far from the mainstream, it makes it nearly impossible for the political system to deal constructively with the country’s challenges."

Yep.