For those who haven’t completely lost their ability to be appalled, Tucker Carlson’s smears this week of Illinois senator Tammy Duckworth can fairly be described as shocking.

He called a Purple Heart recipient a “moron” for suggesting that it was worth discussing the idea of removing monuments of George Washington, the United States’ first president and a slave owner.

Carlson, who has never served in the military, called this veteran, who lost both of her legs fighting in the Iraq War, a fraud, a vandal and — maybe most remarkably — a coward.

Jabbing away with his trademark combination of outrage and glee, he described her as “a deeply silly and unimpressive person,” grouping her among those who “actually hate America.”

Duckworth, a potential Democratic vice presidential nominee, delivered a memorable comeback on Twitter without matching him insult for insult: “Does @TuckerCarlson want to walk a mile in my legs and then tell me whether or not I love America?”

Why would Carlson stoop so low?

On one level, that’s no puzzle.

It captivates his audience — and that audience is growing. He now has the most popular show on the most watched cable news network, pulling ahead of Fox’s usual ratings winner, Trump whisperer Sean Hannity.

And it’s thoroughly in keeping with Carlson’s recent on-air comments in which he called Black Lives Matter protesters “criminal mobs.”

Nor is it very far afield from his past remarks. Just last summer, Carlson absurdly was referring to the whole notion of white supremacy in America as “a hoax.” (“I’ve never met anybody, not one person who ascribes to white supremacy,” he offered as dubious proof. “I don’t know a single person who thinks that’s a good idea.”)

But, over a long career, Carlson — who doesn’t lack intelligence or talent — has had his moments of enlightenment.

He had the good sense in late winter to break with most of the Fox News pack and fully recognize the deadly threat of the novel coronavirus. He even made a trip to visit the president in Mar-a-Lago to try to break through President Trump’s apparent denial about its seriousness.

And going further back in history, Carlson earned the respect of other journalists with his reporting and writing chops.

“If you ask his former editors, they’ll say they’re wistful when they think about the old Tucker Carlson,” wrote Lyz Lenz in a 2018 Columbia Journalism Review profile. She quoted Policy Review’s Adam Myerson: “Tucker was an enterprising, hard-working shoe-leather reporter.” Legendary editor Tina Brown, who worked with him at Talk magazine, called him “a tremendously good writer.”

But he’s been heading steadily downward for years, especially since his arrival at Fox News: a transformation feeding endless fascination among journalists and commenters, as John Harris documented in a Politico piece titled, “Why are writers and editors so obsessed with Tucker Carlson?”

They’re obsessed in part because it’s bizarre watching someone so promising fall so far, and seeing talent spent on such ugliness.

Having co-founded and edited the conservative website the Daily Caller, he was pretty far along this partisan trajectory by the time he started hosting “Tucker Carlson Tonight” in 2016.

And, after moving into Bill O’Reilly’s coveted 8 p.m. slot after O’Reilly’s show was canceled in 2017, Carlson really dug in. He was fully on board with Fox News’s politically expedient coverage and commentary of the dreaded “caravan” in the weeks leading up to the 2018 midterm elections.

Undocumented immigration has made the United States “a fallen country,” he claimed at one point.

Speaking about the migration of people, presumably including some asylum seekers, through Mexico to the United States, he wielded fearmongering: “This is an invasion, and it’s terrifying.” (This alarmist coverage, not so curiously, dwindled after Election Day passed.)

All of this is why advertisers, such as the Walt Disney Co., ­T-Mobile and others, have fled Carlson’s show, not wanting association with his escalating, often hateful rhetoric.

Nearly 40 percent of Carlson’s advertising these days comes from his stalwart friends at MyPillow, headed by Mike Lindell, a Trump supporter who likes to use the expression “all lives matter” — a well-understood swipe at the reform-oriented message of the Black Lives Matter movement.

So, perhaps, the criticism of Duckworth should be no huge surprise. Yet it may well be the worst example yet of how far Carlson has fallen.

That this incendiary swill is so well-received by his audience — if not by skittish or principled advertisers — makes me wonder: Do 4 million Americans really approve? Or are they just watching to get their nightly outrage on?

Either way, the show’s popularity is likely to encourage him and keep his network bosses from reining him in.

We’re about to find out whether there really is a floor of decency that even Carlson won’t sink below. I’m not betting on it.

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For more by Margaret Sullivan visit wapo.st/sullivan