Stocks extended losses by mid-afternoon Tuesday in a weaker start to November as uncertainty over the upcoming presidential election weighed on Wall Street.

The S&P 500 was down 1.3%, the Dow Jones Industrial Average fell 1.1%, and the Nasdaq declined 1.4%. The S&P 500 has fallen for the past six sessions.

Conflicting polls created uncertainty over the outcome of the U.S. presidential election. An NBC/SurveyMonkey poll showed Democratic presidential candidate Hillary Clinton's 6-point lead remained after concerns over her emails were raised again on Friday. A separate ABC poll showed Republican presidential candidate Donald Trump with a 1-point lead. U.S. voters will head to the polls on Nov. 8.

"The reopening of the Clinton email investigation indeed raises the possibilities of a market re-enacting the scenario from the Nixon years, where uncertainties created an unstable market notwithstanding a solid economic environment," said Peter Cardillo, chief market economist at First Standard Financial. "While no one will be able to predict the outcome, one thing is certain: Uncertainties are the markets' worst enemy. In other words, it's time to be cautious."

Crude oil prices fell on Tuesday as worries over a deal among major oil-producing countries persisted. Goldman Sachs added to a chorus of concern on Monday evening after warning that prices could plummet if the Organization of Petroleum Exporting Countries fails to enact a production freeze agreement at a meeting in late November.

A deal looks more and more distant, analysts wrote in a note, with "weakening oil fundamentals warranting oil prices in the low [$40 a barrel range] in our view if OPEC is unable to deliver a convincing agreement."

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