The chance of a recession in 2019 is higher than it's been at any point since Donald Trump became President, according to CNBC's Fed Survey

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The Dow Jones stock market has had its worst December since, wait for it, the Great Depression . (Stocks rallied several hundred points on Tuesday morning.)

The warning signs are ominous. The roaring economy appears to be flagging or, at least showing signs that the upward momentum of the past year-plus is receding considerably.

All of which poses a very interesting political problem for Trump. He has built his entire presidency on the idea that his economic policies, learned and honed in the private sector, are succeeding beyond anyone's greatest expectations.

Asked last week by Fox News' Harris Faulkner whether he felt pressure to keep the strong economy going, Trump responded : "Well, I don't think there is any pressure. Everyone's doing so well. I mean there's not a lot of pressure."

Which, um, judging by the analysis of Greenspan (and others) and the recent performance of the stock market, may well be a bit of whistling past the political graveyard for Trump.

After all, there's no debate that even as he is broadly unpopular in the country, Trump's ratings on the economy remain quite positive -- and have kept his overall ratings from totally collapsing.

In a CNN-SSRS poll conducted earlier this month, just 39% approved of the job Trump was doing as President while 52% disapproved. That's a striking contrast to Trump's ratings on how he has handled the economy in that same poll; 49% approve, 41% disapprove.

The economy is the only issue -- of six CNN asked about -- where more people approve of how Trump is doing than disapprove. And remember, his positive ratings on the economy come in a poll where his overall disapproval number is 13 points higher than his approval number.

Traditional political thinking is that presidents get too much credit when the economy is good and too much blame when it's bad. (The thinking behind that conventional wisdom is that presidents have only limited ability to affect the economy -- for good or for ill -- no matter what policies they put in place.)

Trump has, to date, bucked that conventional wisdom -- to his detriment. Despite the relative strength of the economy, Trump's overall job approval ratings have been mired in the low 40s since almost his first day in office. It as though people who don't like Trump dislike him so much that they are entirely unaffected or swayed by the strong economy. If the economy turns or slows, there's a 100% chance these people will turn even harder from Trump heading into his reelection race.

The more important question is how Trump's political base would respond to a worsening economy. While much has been made of Trump's supposed high floor in polling due to the loyalty of his supporters -- and how little outside events seem to affect their view of the President -- it remains to be seen whether a significant slowdown in the economy (or even a recession) might lower that floor, cutting into even the ranks of his most die-hard backers.

Such a development would be a death knell for Trump's prospects in 2020. And to be clear, we're not there yet. Not even close.

But for Trump to have a real chance of winning a second term -- given how deeply personally unpopular he is -- he badly needs the economy (or at least voters' perception of the economy) to be strong. Which seems like less of a sure thing than it did a few months ago.