When the economy was humming along with only 3.9 percent jobless in September of 1956, Dwight Eisenhower’s approval rating stood at 68 percent. Richard Nixon had a 63 percent approval rating when unemployment reached that same level in January 1970. Bill Clinton’s approval was 58 percent positive when unemployment hit 3.9 percent in October 2000. Yet today, with unemployment again hitting that same low rate, Trump’s approval stands at just 39.8 percent, according to an average of major polls. The famous adage “it’s the economy, stupid” is generally a reliable guide to the political fortunes of modern presidents – except when it isn’t. Consider Lyndon Johnson. In November of 1967, when United States unemployment was at 3.7 percent, LBJ's approval was just 42 percent. His problem? External shocks: the deadly slog of the war in Vietnam, riots burning out the core of major US cities. In Trump’s case, the external shock is arguably him. Republicans see him as a change agent who makes liberals and reporters uncomfortable. But many independents and most Democrats view him as someone with little regard for the truth or the norms of democracy.

Here are some numbers that are good: 4.2, and 3.9. The first is the growth in US Gross Domestic Product for the second quarter of 2018, at an annualized rate. The second is the unemployment rate for August. Both are evidence of a really, really strong economy – though they’re not as historically unique as President Trump sometimes boasts.

Here are some numbers that are bad: 39.8, and 82. The former is the percentage of Americans who approve of the job Mr. Trump is doing, according to an average of major polls at time of writing. That’s ... low. The latter is the percentage chance that Democrats will win control of the House of Representatives in November’s midterm elections. That’s high, uncomfortably so if you’re an elected Republican lawmaker. (Both of these are from the data journalism site FiveThirtyEight.)

So here’s the question: Why isn’t the demonstrably excellent economy making Trump more popular? Does this mean the famous adage about American politics, “It’s the economy, stupid,” is no longer true?

Well, we can’t really tell how the economy is affecting Trump’s numbers, for one thing. It could be keeping them higher than they otherwise would be. And the data show “it’s the economy” remains a reliable guide to the political fortunes of presidents in the modern era.

Except when it isn’t.

Let’s step back a bit and try to put Trump’s numbers in a historical context. We’ll use the unemployment rate, since it’s a simple number, it’s good for Trump, and it correlates pretty well with presidential approval ratings over time. (For the most part.)

The current rate is 3.9 percent, as we noted above. When it hit that level for other presidents, what was their approval rating? Philip Klinkner, a political scientist at Hamilton College, tweeted out some numbers on this recently: Dwight Eisenhower had a 68 percent approval rate when the economy was humming along with only 3.9 percent jobless in September of 1956. Richard Nixon had a 63 percent approval rating when it reached that level in January 1970. Bill Clinton’s approval was 58 percent positive when unemployment was 3.9 percent in October 2000.

Those ratings were all a lot better than Trump’s current number.

But then there’s LBJ. In November of 1967, US unemployment was 3.7 percent, and Lyndon Johnson’s approval was 42 percent. That’s higher than Trump’s current number, but barely.

What was LBJ’s problem? External shocks. The deadly slog of the war in Vietnam dominated front pages. Riots were burning out the core of major US cities. Some white voters disapproved of the administration’s push for civil rights. At times, Washington seemed to be losing control of the country, or at least to be out of touch. Johnson’s job approval peaked at around 80 percent when he took office following John F. Kennedy’s assassination, but then basically began a years-long slide, bottoming out at 35 percent in August 1968.

The Christian Science Monitor/File President Lyndon Baines Johnson in 1965. Presidential approval ratings in the modern era tend to track closely with the economy – except when they don't.

In Trump’s case, the external shock is arguably Trump himself.

Republicans approve of Trump’s personality and actions in office, seeing him as a truth-teller who makes liberals and mainstream news reporters uncomfortable, and who is taking steps to do what the country has long needed. Many also believe the media aren’t covering positive news enough. In the latest CNN poll, 82 percent of GOP respondents approved of the job the president is doing.

But many independents and most Democrats view Trump as someone with little regard for the truth or the norms of democracy. The same CNN survey has his approval rating among self-described independents at 31 percent. Among Democrats, it’s five percent. Talk about a partisan split.

So, what are the implications for the “It’s the economy, stupid” truism? The phrase, of course, was made famous in Bill Clinton’s 1992 presidential run, when force-of-nature aide James Carville hung a sign to that effect in the campaign war room.

Well, a few years ago, in the pre-Trump era, Pew Research took a thorough look at how economic numbers affect presidential approval, and concluded “the public response to all [modern] presidents has been shaped to some degree by rising or falling unemployment.”

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But the correlation is closer when unemployment gets worse – and particularly when it gets worse precipitously. When it gets better, and changes more slowly, the relationship between the measures is murkier.

Political scientists still consider the economy one of the “fundamentals,” those underlying forces that can affect the outcomes of American politics more profoundly than most actual campaigns. But Trump has clearly upended some of their calculations on this subject, as he has so many other things.