How much better would the market perform under a President Clinton than a President Trump?

Consider the rise in stock futures. At first blush, an increase of less than 1 percent doesn’t sound like much. But it points to something much bigger. After all, if a relatively small decline in the likelihood of a Trump presidency led to a modest stock rally, then a larger decline in Mr. Trump’s electoral fortunes would most likely lead to a larger market reaction. A surprise win for Mr. Trump would probably set off a substantial market correction.

We can be a bit more precise. According to political prediction markets, the odds of a Trump presidency fell by nearly six percentage points during the debate. By contrast, the difference between a certain Trump loss and a certain Trump win — by definition, a 100-percentage-point change in probability — is around 16 times as large as that six-percentage-point drop. It follows that the difference in the value of stocks under a President Clinton versus a President Trump is 16 times as large as Monday night’s stock market shift.

Putting the pieces together (multiplying that 16 by the percentage point rise in the S.&P. 500 value), this suggests that the market expects stock prices to be 10 to 12 percent lower if Mr. Trump wins than they will be if he loses.

This is a big difference.

What does it all mean? When economists conduct “event studies” like this, they typically do so in the hope that movements in stock prices reflect the informed bets of traders trying to assess the future profitability of the businesses they’re buying or selling. If that’s right, then Wall Street is telling us that if Mr. Trump is elected, it expects the profitability of America’s largest businesses to be about 10 to 12 percent lower on average in the future.

This is all the more remarkable because it occurs despite Mr. Trump’s having promised an enormous profit-raising corporate tax cut. Either the markets don’t believe he’ll deliver on these tax cuts, or they believe that the rest of his economic program will do enough harm to more than offset the benefits of lower taxes.