Washington (CNN) - President Donald Trump isn't in a great place to win a second term in 2020.

His overall job approval rating has hovered in the low 40s -- and even upper 30s -- for the entirety of his first two years in office. Polling in critical swing states in the Midwest and Sun Belt suggests his electoral map is shrinking rather than expanding compared with 2016.

And yet and yet and yet, there's this: 71% of people in a new CNN-SSRS national poll think the economy is either "somewhat good" (45%) or "very good (26%). That's the highest rating for the economy in a CNN poll since February 2001. Which is a looooong time.

It's not only that people believe the economy is strong, either. It's that they give Trump credit for it. A majority (51%) in the CNN-SSRS poll approve of the job Trump is doing with the economy, while 42% disapprove. (It's the only issue -- out of the five asked about -- where more people approve than disapprove of how Trump is doing; Trump's overall approval rating in the poll is 42%.)

That economic message could well be Trump's secret sauce come 2020. In strong economies -- or economies perceived as strong -- voters tend to reward the incumbent president, just as they punish the president when the economy is faltering. Trump could well run on this basic argument: You may not like me or my style of politics. But look at the results! Is now really the time to change presidents?

In fact, he appears to be laying the groundwork for just such an argument.

"How do you impeach a man who is considered by many to be the President with the most successful first two years in history, especially when he has done nothing wrong and impeachment is for 'high crimes and misdemeanors'?" he tweeted last week.

Now. The heath of the economy could change over the next year-plus. Last month's jobs report was significantly less rosy than months previous. But at the moment, the perceived strength of the economy is the best 2020 argument Trump has going. By a lot.

The Point: Trump's only chance at a second term hinges on people believing he is responsible for the economic uptick. Even with that he could lose. But without it, he will lose.