Greg Valliere is the chief global strategist at Horizon Investments. The opinions expressed in this commentary are his own.

There were plenty of reasons why the stock market plunged this week — a disastrous press conference by Fed Chair Jerome Powell, fears of a decelerating economy, continued anxiety over a trade war, etc. But there's a wild card that cannot be overlooked: the increasingly erratic behavior of President Donald Trump.

Addicted to Fox TV and its talking heads, Trump has parroted their rants and has had, arguably, the worst week of his presidency. For the first time, Republicans on Capitol Hill — who resolutely defended the president for the past two years — are beginning to defect. They were blindsided by Trump's impetuous decision to pull troops out of Syria and Afghanistan, as were many of Trump's top advisers.

Most of Washington was also blindsided by Trump's reversal on a government shutdown. He had grudgingly agreed to keep the government open, but an avalanche of criticism from Ann Coulter, Rush Limbaugh and others persuaded him to dig in on a border wall. Most senators, who were preparing to leave town for the holidays, were stunned.

We have argued for the past year that Trump's tweets and controversies were mere background noise for the markets — not a big deal. But the spectacle of a president acting so erratically has to concern investors, who already have been whipsawed by Powell. Perhaps more importantly, Trump no longer is the favorite to win re-election in 2020.

The GOP's disastrous performance in last month's elections in the Rust Belt — where Trump essentially won the presidency in 2018 — persuades us that it will be much more difficult for him to win 270 electoral votes. And the deep antipathy toward Trump from college-educated suburban women will make a big difference in the next election.

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