THE MORNING PLUM:

Prediction: If Republican Rick Saccone scrapes out a close win in tomorrow’s special election for a House seat in southwestern Pennsylvania, Donald Trump will claim it was all because of Donald Trump. But if Saccone falls just short, Donald Trump will claim it was all because Saccone didn’t sufficiently emulate Donald Trump.

But either way, it will be very bad news for Trump. This isn’t just because this election is deep in the heart of Trump country. It’s also because the failure of the Trump/Republican argument to prevent this contest from being so close also carries ominous signs for the GOP this fall.

In the race’s final days, much of the GOP’s messaging appears focused not so much on the Trump/GOP tax cuts, or even on Trump’s tariffs, but rather on immigration, crime and Nancy Pelosi. An outside group allied with the House GOP recently launched spots slamming Democrat Conor Lamb as a “Pelosi liberal” and for allegedly supporting “sanctuary cities and amnesty for illegals.” The National Republican Congressional Committee has recently released ads that slam Lamb, a former prosecutor, as soft on gun traffickers. A super PAC allied with Trump has an ad that mentions the tax cuts but talks more about “Pelosi liberals.”

A Democratic strategist working with the Lamb campaign who tracks ad buy information tells me that these ads represent the main spending on the GOP side in the closing days. Indeed, last week, Dave Weigel and Josh Kraushaar both reported that Republicans had previously aired ads touting the tax cuts but cycled them out of the messaging, because, as Kraushaar put it, they were “barely moving the needle in the district’s working-class confines.”

The reason the downplaying of Trump’s tax plan — and the emphasis instead on hot-button issues such as immigration — matter can be found in the makeup of this district. This is a place where Trump’s claim that his tax cuts are good for working people should carry weight. Trump won it by 20 points, and it has many of the sort of working-class white voters who apparently looked to Trump as an economic savior. But it turns out that this fact may explain why the tax cuts are not sufficiently resonating.

‘More a populist district than a conservative district’

“This is more of a populist district than it is a conservative district,” Mike Mikus, a Democratic strategist from southwestern Pennsylvania who ran a previous race in that district, told me today, referring to the leanings of voters in steel and coal country in places such as Washington, Westmoreland and Greene counties, which Trump carried in 2016 by 61-36, 64-33 and 70-28, respectively. Those are huge margins, a reminder that this district is mostly deep, deep, deep Trump country.

Still, Mikus argued to me that the district is more “economically diverse” than is commonly understood, with a large chunk of the more educated voters coming from the suburbs of Pittsburgh in Allegheny County. But this, too, highlights something important. Republicans hope that the tax plan will also stanch losses among college-educated and suburban white voters who might stomach Trump’s excesses and vote their bottom lines instead. Yet it seems clear that Lamb will do well in those areas, perhaps mirroring places such as Virginia, where Trump’s sexist, racist and xenophobic provocations are shifting white suburban voters toward Democrats and supercharging turnout among them to boot.

And if anything, Trump and Republicans are doubling down on those cultural provocations. In addition to the ads referenced above, Trump’s rally in Moon Township over the weekend featured full-blown race-baiting and authoritarian appeals, with brutal attacks on the media and on a prominent African American congresswoman as “a low IQ individual.” This, apparently, is what is necessary to excite Trump country and get those Trump and GOP voters out — even amid this awesome Trump economy.

To be clear, Saccone could still win. But if Lamb keeps it close, it will still show that the Trump/GOP tax cuts aren’t enough to get educated suburban white voters to put aside their distaste with Trump in the numbers Republicans need. What’s more, this race is shaping up as a referendum on whether Republicans can hold Trump’s blue-collar white base, including the disaffected union Democrats who defected in 2016, in Trump-like numbers. But Mikus suggested to me that Lamb, who has campaigned on a pro-labor message, may be showing how Democrats can begin to reverse that trend, because even in Trump country, he’s “bringing some of those voters home.”

If so, this will show that candidates matter, to be sure. But it will also show that Trump’s agenda — which is basically Orthodox Ryanism overlaid with reckless protectionism in the form of the tariffs, plus a lot of bombast — isn’t enough to prevent good Democratic candidates from bringing back some of those voters. And that could have an impact on Democratic chances of winning the House this fall, since that turns in part on whether they can put some of these more working-class districts in play.

* BAD NEWS FOR GOP IN PENNSYLVANIA: CNN’s Harry Enten points out persuasively that even if Lamb loses but keeps it close in PA-18, it will bode badly for Republicans:

History tells us it’s far more significant to look at the margin between Lamb and Saccone, not at who ultimately wins or loses. And if we’re only looking at the margin, it’s pretty clear that the result in Pennsylvania’s 18th could very likely end up being bad news for Republicans. … the race shouldn’t be close even if the national environment was neutral. Saccone should be winning by double digits.

As Enten notes, the swing in special elections tends to foreshadow results in midterms. So if Lamb falls short by single digits in a district Trump carried by 20, that’s a big swing toward Democrats.

* MUELLER MAY KEEP OBSTRUCTION DECISION SECRET: Bloomberg reports that special counsel Robert S. Mueller III is looking to interview Trump and Donald Trump Jr. to wrap up the obstruction-of-justice piece of his inquiry. But:

[Mueller] may set it aside while he finishes other key parts of his probe, such as possible collusion … Mueller may calculate that if he tries to bring charges in the obstruction case — the part that may hit closest to Trump personally — witnesses may become less cooperative in other parts of the probe, or the president may move to shut it down altogether.

Of course, if Mueller keeps his conclusions on this front secret over coming months, that will also prompt Trump to grow more and more agitated.

* MUELLER IS FOCUSED ON KEY OBSTRUCTION MOMENTS: The Bloomberg report also tells us this:

Mueller is said to be focused on three main episodes: Trump’s firing of FBI Director James Comey last May; the drafting of a misleading statement about the purpose of a June 2016 meeting between Don Jr., Trump’s son-in-law Jared Kushner and a group of Russians at Trump Tower; and the disclosure that Trump considered firing Mueller last June.

And Mueller’s investigators have already interviewed multiple top officials with firsthand knowledge of these episodes.

* TRUMP WILL PUSH FOR ARMED SCHOOL EMPLOYEES: White House aides told reporters last night that Trump will push forward with his plan for more armed school employees, and will establish a commission to study other ideas. But:

The proposals did not include Mr. Trump’s repeated call to raise the age for buying certain firearms from 18 to 21, an idea that has been opposed by the National Rifle Association. … The president will also push for Congress to pass a bill … to improve reporting to the national background check system — a modest step backed by the N.R.A.

That last one is not nothing. But you can bet Trump’s call for a comprehensive closing of the holes in the background-check system will also vanish.

* A COMMISSION FOR GUNS, BUT NOT FOR DRUGS: With the White House creating a commission on guns, The Post points out that Trump has mocked the idea of a commission to study the drug epidemic, claiming it can only be solved with “toughness”:

Administration officials demurred Sunday night when asked why Trump found commissions an inadequate response to the drug epidemic but an appropriate way to respond to gun massacres. “There are not going to be one-size-fits-all approaches and solutions, and I think that that is a very cogent argument for having a commission,” said a senior administration official.

Weird that Trump’s “toughness” appears to have disappeared on guns, now that the NRA said, “Boo!”

* TRUMP’S MESSAGE TO HIS BASE: E.J. Dionne Jr. makes an important point about the message Trump intended his tariff plan to send to his base:

His working-class supporters … have little to show for his presidency. … “Trump Country” is not experiencing the renaissance he predicted, in part because he could not have kept his outsize promises in the first place. While Trump scaled back the reach of the tariffs from his original proposal, they still sent a loud message to his straying base: Remember the old me; I’m still here.

Working people are already shrugging about Trump’s awesome tax cut (actually a huge giveaway to the rich), so maybe the continued deportations and racial provocations will keep them persuaded on that point.

* AND MNUCHIN THINKS TRUMP IS REALLY ‘FUNNY’: Chuck Todd grilled Treasury Secretary Steven Mnuchin about Trump’s Saturday rally, at which he attacked the press and said a prominent black congresswoman is a “low IQ individual.” Mnuchin stammered:

“You know he likes to put names on people. … the president likes making funny names….He’s using these vulgarities in the context of a campaign rally and obviously there were a lot of funny moments on, on, on that rally.”

It’s hard to say what’s more pathetic — the idea that Mnuchin might actually find this stuff funny, or the fact that he feels the need to say this to his audience of one in the first place.