(Susan Walsh/AP)
Opinion writer

This morning, President Trump sent out a series of tweets touting what he regularly says is the best economy in the history of the United States. When you say that to an economist of any political party, they’ll either shake their head sadly or burst out laughing.

But the question I’d like to address is why, when the economy is actually doing well by many measures, the president insists on lying about it in an attempt to make it seem even better than it is.

After all, he’s got a pretty good story to tell using actual facts. So what’s with the lying? The answer isn’t as simple as it might seem.

Let’s start with this:

Even if it were true, this would be nothing but an arbitrary factoid; the rate of change in GDP and the unemployment rate are two entirely different things. But of course, it’s not true, as economist Justin Wolfers pointed out.

In fact, since 1948 (the earliest data we have), the quarterly GDP rate has exceeded the unemployment rate 64 times. His assertion that it hasn’t happened in “over 100 years!” is particularly weird given that there are no data from back then; it’s much like his assertion that he has better poll numbers than Abraham Lincoln. (Scientific polling wasn’t developed until the mid-20th century.)

Trump also likes to claim that everyone said reaching 4 percent growth was impossible and he’s done it, which is also false. Many said that his promise of sustained 4 percent growth would be impossible, not that we’d never see a single quarter with that level of growth; it happened multiple times while Barack Obama was president.

So why is it necessary for Trump to not simply tout the good things about the economy but to layer fantastical falsehoods on top? The easiest answer is that this is just what Trump does: He’s a salesman, and just like always, he’s selling himself. Part of the Trump brand is that whatever he’s associated with is the best, the biggest, the most luxurious, the most successful. “The Apprentice” began with Trump saying, “My name is Donald Trump, and I’m the largest real estate developer in New York.” It wasn’t true then; in fact, it was never true, not even close. But he’s so used to saying that whatever he touches is the absolute top that he can’t help but make the same claims as president, no matter how ludicrous they might be.

That’s part of the story, but I don’t think it’s the real reason that Trump has become so desperate about insisting this is the best economy ever. It’s not just salesmanship. I think it’s because he’s angry and hurt that he isn’t getting the credit he thinks he deserves.

And that isn’t a completely crazy thing for him to feel. After all, for as long as there have been polls, we’ve identified a close relationship between economic performance and presidential approval. Other factors can have an effect, like whether there’s a war or scandal happening, but by and large, if unemployment is low, most presidents will see high approval ratings.

Yet right now we have a highly unusual situation: Unemployment is extremely low, and the president’s approval ratings are also extremely low. Polls of late show his approval around 40 percent or lower; in a Quinnipiac University poll released Monday he’s at 38 percent, despite the fact that 70 percent in the same poll say that the economy is excellent or good, matching the highest score Quinnipiac has recorded on that question.

That must drive Trump absolutely nuts.

In one way, it’s not entirely his fault. We’ve known for a long time that partisanship affects economic perceptions: When there’s a Democrat in the White House, Republicans are more likely to say the economy isn’t doing well, and vice versa. Not only that, as polarization increases and parties come more to define people’s identities, they may be less willing to say they approve of a president from the other party, no matter how well the economy is doing. That puts a ceiling on the approval any president can get except in the most extraordinary of situations, like the aftermath of the Sept. 11, 2001, attacks.

And then there’s Trump himself. He’s so personally and politically repugnant that a good portion of the public, perhaps a majority, is not willing to overlook the awful things he does and says no matter what economic indicators may be in positive territory.

It’s often said that presidents get more credit than they deserve when the economy is good and more blame than they deserve when the economy is bad. But right now, the economy is good and Trump isn’t getting the credit. Which means we might finally have it right.