Donald Trump likes to tout his ironclad support among Republican voters. And the media often play along with the idea that he could, as he has famously said, shoot someone in the middle of Fifth Avenue and not have any of his backers actually care.

The political reality, however, is far bleaker for the president as we head into 2020. The news of Joe Walsh underscores that. Joe Walsh, yes, that Walsh, who is challenging Trump in the Republican primary.

So, what to make of this?

On the one hand, Walsh is obviously not going to win the Republican nomination for president. He served a mere one term in Congress. He may not raise much money and thus may not be able to run anything like a conventional campaign.

And now, the other hand. Walsh is most definitely no hero to anybody outside the hard-right fever swamps—his history of racist tweets about Barack Obama and other topics, which George Stephanopoulos pressed him on, are evidence enough of that. (He did at least admit they were wrong.) But more recently, he has been a Rottweiler biting Trump’s corpulent ass. I’m sure you’ve seen some of his tweets over the years. They’re vicious. So the guy ain’t afraid. And though he’s a pretty rabid right-winger, most of his tweets about Trump read as if they could have been written by anybody with politics more like mine—they’re almost entirely about what a lying, unprincipled hulk of condemned meat Trump is. He has a large Twitter following (207,000) and a nationally syndicated radio show.

And it was Trump’s mendacity and boobery Walsh emphasized on Sunday, not his ideology, such as it is. “I’m running because he’s unfit; somebody needs to step up and there needs to be an alternative. The country is sick of this guy’s tantrum—he’s a child,” Walsh said. In other words, his case, it seems, isn’t going to be ideological, but characterological.

Ah, but you say, Republicans don’t want to hear that. To which I say, well, most of them don’t, true. But I’ve noticed something interesting happening lately. Trump’s numbers among even Republicans are starting to drop, a little. That is something to watch—and it, combined with feral attacks from a primary opponent who, every time Trump lies, will stand up and say “that’s a lie,” and who will be speaking directly to Republican voters about why it’s a disgrace to have Donald Trump in the White House, could actually add up to something.

The polls. Trump of course constantly brags about his record support among Republicans. The figure he’s been using lately is 94 percent. He says this so often, plucking the friendliest, most Rasmusseniest number out of the air that he can find, and cable news just repeats it so that it’s now accepted by everyone that Republicans adore him. Right?

But just lately, a couple polls suggest a softening—that a growing number of Republicans are finally starting to admit to themselves that Trump shouldn’t be running a Best Buy, let alone the country. A Monmouth poll last week found him at 84 percent among Republicans, and an AP-NORC survey put him at 79 percent (both are discussed here).

That still sounds high, maybe. But let me break these percentages down into raw numbers, and you’ll see why 80 percent approval in a president’s own party can spell trouble.

There are roughly 230 million adult citizens in the country. About three-quarters of adult citizens register to vote, maybe a little more. So let’s say there are 175 million registered voters in the United States.

Now, party registration fluctuates, and it’s all somewhat imprecise because only 31 states require party affiliation of voting registrants. Pollsters like Gallup and Pew regularly ask people whether they consider themselves Democratic, Republican, or independent. These numbers shift with events in ways you’d expect: The Republican number bumped up after 9/11, the Democratic figure increased after Barack Obama’s election. But usually, independents are around 40, Democrats are in the low 30s, and Republicans are in the high 20s, something like that.

So let’s say 28 percent of registered voters are Republican. Twenty-eight percent of 175 million is basically 50 million. Okay, now let’s say by election time, Trump is at 80 percent among Republicans. Well, 20 percent of 50 million is 10 million. That means that 10 million Republicans can maybe be persuaded to vote against the man. Or to withhold their support from him and stay home.

Given how close the vote totals were in 2016 in a number of states, these 10 million could make an enormous difference. Florida, 110,000 out of 9 million cast; Pennsylvania, 44,000 out of nearly 6 million; Wisconsin, 22,000 out of 2.8 million; Michigan, 11,000 out of 4.5 million. If there are 10 million anti-Trump Republicans in November 2020, isn’t there a decent chance that 11,000 of them live in Michigan?

Out of curiosity I went back and looked at the exit polls over the last 20-plus years’ worth of elections. Trump got 88 percent of Republicans in 2016. Mitt Romney got 93 percent in 2012. John McCain got 90 percent in 2008. George W. Bush got 93 percent in 2004 and 91 percent in 2000.

Then we go back to 1996, when Bob Dole ran against Bill Clinton. Dole got…80 percent of Republicans. Yes, party loyalties were less metastasized then, but whatever the explanation, the fact is the fact. Dole won just 80 percent of Republicans, and he lost—by 8.5 percent, 8 million popular votes, and a whopping 220 Electoral College votes.

So 80-percent support in a president’s own political party may sound high at first glance, but it’s not. It’s shaky territory. I’m not saying Trump is there quite yet. But there are encouraging signs that he’s headed in that direction. And Walsh obviously wants to take him there and doesn’t care if he costs Trump the White House and maybe even hopes he does.

This doesn’t mean anyone should forgive Walsh his own racist history. But if he contributes to Trump’s defeat, that should be worth a few atonement points. And who knows, maybe he'll become a better person. As I've said before: They used to say a conservative was a liberal who’s been mugged by reality; today, and in the future, a liberal is a conservative who’s been Trumped by it.