Orkin, the exterminator, reports that Washington is the fifth “rattiest” metropolitan area in the United States. But because three of those ahead of us (Chicago, New York and Los Angeles) have larger human populations, it would appear that Washington is vying with Nancy Pelosi’s San Francisco for most rats on a per capita basis. Or would that be a pro rata basis?

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Most suspect that the culprit is the common brown rat (Rattus norvegicus), not to be confused with the black rat (Rattus rattus), the Himalayan field rat (Rattus nitidus) or others.

I have a different theory. The capital has, since Jan. 20, 2017 (give or take), been overrun by another species: Rattus lutreolus, the swamp rat. This species feeds mostly on political action committee contributions, and though native to Australia, was recently spotted living in a lobbyist’s condo at below-market rates.

The rat problem has come to the attention of Trump himself, who recently expressed his pleasure that his aides wouldn’t be “a John Dean type ‘RAT.’ ” A few weeks later, Trump’s former campaign manager turned into a rat.

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It’s spreading like the plague.

In the days since John McCain died, there have been increased sightings of Rattus erraticus. This species, native of South Carolina, exhibits submissiveness toward certain large male rats and is unpredictable in behavior. One Rattus erraticus specimen, Lindsey O. Graham, spent two years defending the integrity of the same Justice Department he now claims is staging a “bureaucratic coup” against Trump.

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But the Brett M. Kavanaugh confirmation battle is really bringing the rodent from its burrow. Here’s a taxonomy to better understand Washington’s newest residents:

At the top of this animal kingdom is an extremely large species with yam-tinged fur. Under the scientific name Rattus potus, it is highly aggressive with other rats (except for immediate family members), often for no apparent reason and even when not in its self-interest. Such behavior was exhibited by Trump when he ignored advisers and suggested Kavanaugh’s alleged sexual assault of Christine Blasey Ford was not “as bad as she says.”

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The behavior exhibited by Rattus potus has prompted similarly odd comportment among lesser species, including:

Rattus slanderus. Kavanaugh pal Ed Whelan was put on leave by his right-wing think tank after he sought to pin the assault — falsely but publicly — on another man.

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Rattus imbecilis, now listed as electorally endangered. Specimens include Dean Heller , who proclaimed the allegation against Kavanaugh to be a “hiccup”; GOP Senate candidate Kevin Cramer of North Dakota, who called Ford’s allegation “even more absurd” than Anita Hill’s because Ford’s involved teenagers; and Ralph Norman , who joked that Ruth Bader Ginsburg claimed “she was groped by Abraham Lincoln.”

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Rattus misogynus. Commonly known as the “naked male rat,” Rattus misogynus is exemplified by Steve King , who warned an audience that “no man will ever qualify for the Supreme Court again.”

Rattus paranoius. White House counselor Kellyanne Conway told CBS News on Monday that “this is starting to feel like a vast left-wing conspiracy.”

On the opposing side of the Kavanaugh affair are two other species:

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Rattus opportunis. This species, native to New York, nests in green rooms and spends hours grooming itself. Specimen Michael Avenatti, the Stormy Daniels lawyer, injected himself into the Kavanaugh matter by claiming without any substantiation to “represent a woman with credible information regarding Judge Kavanaugh.”

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Rattus spartacus. This species is known for a highly verbal behavior rodentologists describe as “grandstanding.” One specimen appeared during the initial Kavanaugh hearings, when Cory Booker announced he would martyr himself by releasing documents it turned out were already cleared for public release.

The nominee himself is described in the New Yorker by a Yale roommate as being “frequently, incoherently drunk.” This attribute is common among the species Rattus inebrius.

Rattus inebrius enjoys a symbiotic relationship with the powerful, beady-eyed species Rattus ginormus, commonly referred to as Mitch McConnell . If Rattus ginormus doesn’t like a Supreme Court nominee, even an impeccably qualified one, he refuses to budge for a year. If he likes a nominee, even one accused of sexual assault, he says “We’re going to plow right through it.”

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He is the biggest rat of all.

Twitter: @Milbank