Wharton finance professor Jeremy Siegel warns that stocks could drop 10% to 20% if the United States and China dig in during trade talks.

Siegel said the market had built in about a 90% chance that trade negotiations would be resolved with China.

Since President Donald Trump’s tweets on Sunday threatening to raise tariffs, the market now projects no more than a 70% chance of a resolution, he told CNBC.

“The strongest thing that Donald Trump has going for him in next year’s election is the economy and the stock market. He cannot afford that to falter,” Siegel said.

“If both sides dig in this market could go down 10% to 20%,” Siegel said.

“It’s a question of what happens on Friday. If it does happen on Friday, what is the retaliation of the Chinese? And that’s totally dominating the market for the next two or three weeks.”