Investors are preparing for a deluge of quarterly earnings reports in the coming weeks. The numbers may not be pretty.

That could be a problem, considering that stocks have held up reasonably well this year despite concerns about a slowing global economy, US trade tension with China and President Trump's looming impeachment inquiry.

Weak corporate profits could be what finally convince investors to start pulling back on stocks, especially if companies start to lower their outlooks for the fourth quarter and 2020.