Stocks suffered a dismal start to the month on Tuesday as conflicting polls bred uncertainty over the upcoming presidential election.

The S&P 500 was down 0.68%, the Dow Jones Industrial Average fell 0.58%, and the Nasdaq declined 0.69%. 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."

West Texas Intermediate crude fell 0.08% to $47.42 a barrel on Tuesday.

Defensive utilities stocks were the worst performers on markets Tuesday. NextEra Energy (NEE - Get Report) , Duke Energy (DUK - Get Report) , Southern Co. (SO - Get Report) , National Grid (NGG) and Dominion Resources (D - Get Report) fell, while the Utilities SPDR ETF (XLU - Get Report) slid 1.8%.

Yum! China Holdings (YUMC - Get Report) climbed in its market debut on Tuesday. Shares added more than 8% after the China operations were spun off from parent Yum! Brands (YUM - Get Report) . Yum China has exclusive rights to KFC, Pizza Hut and Taco Bell in mainland China.

Manufacturing activity in the U.S. in October rose at a faster pace than expected, according to the latest ISM Manufacturing Index. The measure increased to 51.9 last month from 51.5 in October. Analysts expected a reading of 51.7. A comeback in manufacturing was welcome after a higher U.S. dollar and low oil prices stunted growth for months.

A separate reading on the manufacturing sector hit a one-year high as increases in output and new orders drove manufacturing activity. The final Markit PMI reading for October increased to 53.4 in its final reading for the month, up from initial estimates of 53.3.

U.S. construction spending in September showed a surprise decrease. Construction spending fell 0.4% in September, according to the Census Bureau. Analysts had expected a 0.5% increase after spending declined 0.5% in August. Private nonresidential construction weighed on the total as commercial and manufacturing building activity declined. August's number was also revised to show a smaller increase.

In earnings news, Royal Dutch Shell (RDS.A - Get Report) jumped 4% after swinging to a profit in its third quarter. The oil company reported quarterly profit of $1.4 billion, reversing a loss of $6.1 billion in the year-ago quarter. Shell managed to turn a profit as it focused on cost cutting in the face of a prolonged period of low oil prices.

BP (BP - Get Report) also reported a better-than-expected quarter as it stripped down operations to weather a tough climate. Third-quarter profit rose 35% after three straight quarters of net losses.

Pfizer (PFE - Get Report) fell 2% after issuing a disappointing outlook for earnings. The drugmaker said it expects full-year earnings of $2.38 to $2.43 a share, narrowing the upper-range of its guidance from $2.48. Analysts had anticipated full-year earnings of $2.46 a share. Pfizer also missed third-quarter profit, reporting adjusted net income of 61 cents a share, a penny below estimates. Revenue rose 8% and came in roughly in line with consensus.

Shire (SHPG) cut its full-year outlook as costs associated with its acquisition of Baxalta rose. The drugmaker expected a diluted loss per American Depository Share of 70 cents to $1.10 this year. The company had previously expected zero earnings to a net loss of 40 cents a share.

Two-thirds of S&P 500 companies have reported earnings so far this season. Of those that have reported, 71% have exceeded earnings estimates, while 53% have passed revenue forecasts. Third-quarter earnings are expected to grow 3.1%, according to Thomson Reuters, snapping the longest earnings recession since 2009.

In deal news, Gannett (GCI - Get Report) abandoned its bid to purchase newspaper publisher Tronc (TRNC) . The two had failed to reach an agreement on price after months of talks. Tronc shares slumped 12%.

Automakers posted their October sales reports on Tuesday. General Motors (GM - Get Report) breezed past sales estimates over October. The automaker reported a 1.7% decline in unit sales to 258,626, far better than an expected 6.1% decline. Fiat Chrysler (FCAU - Get Report) sales reported a sharper-than-expected decline in October, down 10% compared to an expected decline of 8%. Ford (F - Get Report) postponed its October sales report until later this week. The automaker said the delay was tied to a fire at its corporate headquarters in Michigan. October had two fewer selling days this year than the last.

The Federal Open Market Committee convened on Tuesday morning to discuss monetary policy. The Federal Reserve's policy-making group will conclude the meeting on Wednesday afternoon with a statement, but no press conference is scheduled.

"While the market is widely anticipating the Fed to keep policy unchanged in Wednesday's announcement, the statement itself should hold plenty of clues as to the Fed's objective for rates at the final meeting of 2016," said Lindsey M. Piegza, chief economist at Stifel. "If it's anything like last year, and the Fed intends to follow through with a year-end hike, the language this month will no doubt provide specific guidance; reinforcing expectations for another increase come December."