If this stock market reversal is due exclusively to the virus, then why is United Healthcare down far more than SPX? Why is healthcare as a sector broadly not outperforming? Answer to these questions: the market is digesting a better than 50% chance of Bernie getting the nomination.

Thats DoubleLine Capital Jeffrey Gundlach explaining in an email to CNBC why he believes the stock market has taken such a bruising this week.

The S&P 500 dropped more than 6% over the course of Monday and Tuesday before mounting a slight comeback in Wednesdays trading session.

Maybe this is the dark side of momentum investing, Gundlach wrote. The market goes down in a knee jerk way on the Bernie rise, but the market going down makes Bernies polls go up on his rejection of a market based economy. Which makes the market go down another leg. Rinse and repeat.

This isnt the first time that Gundlach has warned of what a Sanders presidency could ultimately do to the stock market.

If people get more worried about Bernie Sanders and they start to price in his spending programs, then you could really start to see trouble in both bonds and stocks, which could really be on a rough ride, he said earlier this year.

At last check, the Dow Jones Industrial Average DJIA, -4.42% , S&P 500 SPX, -4.42% and Nasdaq Composite COMP, -4.61% were all in positive territory, but were well off their highs for Wednesdays session.

In the latest Real Clear Politics poll for the presidential election coming up in November, Democratic presidential candidate Bernie Sanders beats incumbent President Donald Trump by a margin of 47% to 44%.

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Bond King says were experiencing the dark side of momentum investing and its not because of the coronavirus - MarketWatch