U.S. equities closed sharply lower on Monday, with financials and health care lagging, as market watchers kept a close eye on a key OPEC meeting and looked ahead to a U.S. presidential debate.

"The market has basically priced in a Hillary Clinton victory," said Randy Warren, chief investment officer at Warren Financial Service. "If it's a big victory for Hillary, then no one will watch the next two debates and the market is going to go back to business. If it's a big victory for Trump, then no one will watch the next two debates and everyone is going to be freaking out."