If a recession occurs in 2019, it could spark a 20% decline in stocks.

Brad McMillan, chief investment officer at Commonwealth Financial Network sees the S&P 500 wrapping up 2018 at 3,000, or roughly 3.4% higher than its current level of 2,900.

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.)