Only 720 million? Why not just promise a gazillion? Joe Biden tried selling his child-care tax credit plan yesterday by lamenting that the US tax code provides deductions for racehorses but not for child care. His plan, Biden promised, would put into the workforce six times as many adult women as actually exist in the US at the moment.

It’s tough to get this much wrong in so short a time, but here we are:

You get a tax break for [a] racehorse, why in God’s name couldn’t we provide an $8000 tax credit for everybody who has childcare costs? It would put — it would put seven hundred and twenty million back — million women — back in the workforce. It would increase the GDP, to sound like a wonk here, by about eight tenths of one percent. It would grow the economy.

Even before we get to the math, the current tax code actually does provide deductions for child care as well as dependent care. The child has to be younger than 13 and the benefit is capped at $3000 for one or $6000 for two or more. Biden’s tax credit would give more cash and apply it up front rather than as a deduction, so it’s different — but it’s hardly the gamechanger Biden’s selling. And yes, the tax code actually does provide “tax breaks” for a racehorse investment as well as child care.

But the gaffe here on math is even worse than Guest suggests. According to the BLS, the US has 125.7 million women age 20 or older, with a 59.0% participation rate. That’s below the participation rate of 71.7% for men of the same age bracket, but it’s the highest participation rate for adult women in six years. During the Obama administration, the adult female participation rate fell from 61% when Biden and Barack Obama took office (in the middle of the Great Recession, no less), dropped below 59.0% for good in July 2013, and finished up at 58.3% when Obama and Biden left office in January 2017. August’s 59.0% is the best of the Trump administration so far.

(Worth noting: Men fared slightly worse over the same period. Their labor force participation rate started at 75.2% in February 2009 and declined steadily all during the Obama-Biden administration. It’s been below 72.0% since May 2015.)

So how many adult women are not already in the US labor force? BLS puts it at 51.6 million as of August. It’s tough to get 720 million out of that number, and even tougher to find it among the 2.5 million adult women counted as unemployed but in the workforce. The female unemployment rate is at 3.3% now, more than a full point better than it was when Biden and Obama left office (4.4%, although it was 6.8% when they first took office, too).

By the way, the Trump administration has done a pretty good job of growing the adult-female workforce. They have added 2.3 million more adult women to the workforce since taking office in January 2017. That growth in 2.5 years nearly equals the eight-year growth in the adult-female workforce during the Obama-Biden administration (2.9 million).

Basically, Joe Biden pulled this number from the time-honored political source known as one’s own backside. There’s no way of knowing how many women would enter the workforce with an $8,000 tax credit for child care, but it’s not going to be 720 million, and likely won’t be 720 thousand or 72 thousand either. It’s an idiotic argument buttressed by fantasy numbers in the head of a candidate who had eight years to solve the problem he’s highlighting now as if it just suddenly occurred to him.