Donald Trump made a lot of big promises on the campaign trail, and as president he is doing the same. His new White House website, while still relatively sparse, promises “a bold plan to create 25 million new American jobs in the next decade and return to 4 percent annual economic growth.”

That job number is nice and round — 25 million over 10 years — but it’s also extremely difficult to believe that Trump will be able to pull it off. And though overpromising certainly has its virtues as a political strategy, it’s also risky to make this kind of sharp, numerically precise forecast for your policy results, since it practically invites your opponents to declare you a failure.

Trump’s basic problem is that achieving the goal not only requires creating 25 million job opportunities, it also requires having 25 million new workers to fill those positions. But the size of the American workforce 10 years from now is essentially preordained by today’s population distribution. And unless Trump is planning for a massive expansion of immigration, there simply aren’t going to be 25 million more people available for work over the next decade.

Trump is promising an unprecedented job-creation streak

I went back and ran the numbers, and it turns out there has never been a 10-year streak during which the United States added 25 million jobs. We came close in 1991-2000 and 1992-2001, but neither decade quite reached the 24 million job threshold.

And not only were the 1990s a very unusual moment of labor market health, they were also a time when the working-age population of the United States was growing at a much more rapid clip than it is today. Over the next 10 years, the very large baby boomer cohort is going to continue falling out of the workforce. The also-large millennial cohort is already in the working-age population, and the cohort just behind it that will be leaving school is smaller.

By the same token, the 1990s saw the American labor force augmented by a huge influx of unauthorized border crossings — adding about 2 million people to the workforce between 1995 and 2000. People still overstay visas, cross the border, and otherwise find ways to work illegally, of course. But on net, since 2007 or so the unauthorized workforce has been slowly shrinking.

If you can’t grow the overall working-age population, another option is to employ people who are unemployed or have stopped looking for work. But while such people clearly exist, there simply aren’t nearly as many of them as Trump’s bleak campaign rhetoric would point out.

Twenty-five million jobs over 10 years comes out to about 208,000 jobs per month. This is only slightly higher than the pace of 180,000 jobs per months we’ve seen over the past year. The problem is (according to the Hamilton Project’s jobs gap calculator) we’re only about five months away from reaching pre-recession employment levels if we add 208,000 jobs per month. We’re very likely to see slower job growth than that over the next 115 months of Trump’s pledge — keeping up with population growth at that point would only entail about 90,000 jobs per month.

Beyond all that, the elephant in the room is the possibility of recession. The United States is already enjoying the longest consecutive run of job growth on record, and it would be totally unprecedented to go 10 more years without a downturn.

People might not care — but then again, they might

Overpromising is always a double-edged sword for politicians. Hillary Clinton, tempered by decades in the arena, was unusually averse to big promises — countering Bernie Sanders’s bold social democratic vision with a deeply unorthodox “actually, no we can’t” message of legislative realism. Candidate Trump was an outlier in the other direction, at one point going so far as to say he would fulfill “every dream you’ve ever dreamed.”

The concern elected officials typically have about unrealistic promises is that voters may expect officials to fulfill them and get angry when they don’t.

One line of thought, offered in Slate by Alan Levinovitz, is that Trump won’t pay a price.

“Trump’s rise to power has followed a similar trajectory to that of quacks who peddle panaceas to the desperate — a bizarre and heartbreaking world I’ve long studied,” he writes. “Just like them, Trump will fail to deliver. But his supporters will find a way to exonerate him.”

Most of them undoubtedly will. But this kind of thinking misunderstands the practical operation of presidential politics. The vast majority of the people who voted for Donald Trump in 2016 also voted for Mitt Romney in 2012 — but a small number of extra supporters he picked up is the difference between winning and losing. Four ineffectual, inflation-ravaged years in office didn’t cause the typical Jimmy Carter voter to suddenly turn against him. But dropping from 40 million votes in 1976 to 35 million in 1980 was enough to turn narrow victory into landslide defeat.

Of course that doesn’t mean Trump necessarily needs to hit any precise numerical target. But there’s every reason to believe that many of the people who voted for him — especially the ones who aren’t necessarily Republican Party loyalists or regular voters — really are expecting a dramatic economic transformation that will radically alter the trajectory of their communities. That Trump is carrying these promises forward off the campaign trail and into the White House shows that he believes they are important to maintaining his political support. If it doesn’t pay off, most of his voters will stick with him. But for a new president who won with just 46 percent of the vote, “most” isn’t nearly good enough.