If a recession occurs in 2019, it could spark a 20% decline in stocks.
But in 2019, he's worried about a recession. While it wouldn't be anything as bad as 2008, a few back-to-back quarters of negative GDP growth could spark a bear market in stocks, McMillan said.
A 20% decline from 3,000 would put the S&P 500 at 2,400.
(Editor's Pick. This item originally appeared on Aug. 29.)