One metric has accurately predicted the outcome of every presidential election since 1984 — the strength of the S&P 500 Index.

And right now, the S&P 500 is pointing to a Donald Trump victory on Election Day.

Bloomberg and CNBC noted this week that the stock market's decline does not bode well for Hillary Clinton, the nominee for the incumbent Democratic party.

"Going back to World War II, the S&P 500 performance between July 31 and October 31 has accurately predicted a challenger victory 86% of the time when the stock market performance has been negative," Sam Stovall, chief investment strategist at CFRA, told CNBC.

When stocks are going up, the incumbent party tends to win the White House. But the S&P 500 is down 2.2% since the last trading day of July.

4 PHOTOS Must-win states for Donald Trump See Gallery Must-win states for Donald Trump Florida Republican presidential candidate Donald Trump pumps his fist in the air during a campaign rally at the Collier County Fairgrounds on October 23, 2016 in Naples, Florida. Early voting in Florida in the presidential election begins October 24. (Photo by Joe Raedle/Getty Images) Ohio US Republican presidential candidate Donald Trump speaks at a rally at The Champions Center Expo in Springfield, Ohio, on October 27, 2016. (Photo credit PAUL VERNON/AFP/Getty Images) Iowa Donald Trump, 2016 Republican presidential nominee, speaks during the 2nd annual Roast and Ride hosted by Senator Joni Ernst, a Republican from Iowa, not pictured, in Des Moines, Iowa, U.S., on Saturday, Aug. 27, 2016. Ernst, who in 2014 won the Senate seat vacated by Democrat Tom Harkin when he retired, has turned her Roast and Ride into the conservative answer to the Harkin's legendary Steak Fry fundraiser, which auditioned dozens of presidential candidates over its 37-year history. (Photo by Daniel Acker/Bloomberg via Getty Images) North Carolina Republican presidential candidate Donald Trump speaks to supporters at a rally on September 12, 2016 at U.S. Cellular Center in Asheville, North Carolina. Trump criticized Democratic rival Hillary Clinton for saying that half of his supporters belong in a 'basket of deplorables.' (Photo by Brian Blanco/Getty Images) Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

Still, this election has hardly abided by historical standards. And the downturn in the stock market could actually be a result of election anxiety.

"This time around if the Democrats retain the White House, I will come up with two responses," Stovall told CNBC. "One is that history is a guide but never gospel, and two, the negative performance by the market could be a reflection of the worry of domination that a Democratic sweep would bring."

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As Business Insider's Elena Holodny noted this week, when it comes to markets, the past does not predict the future.

Daniel Clifton at Strategas Research Partners gave Business Insider additional context about this indicator earlier this year:

"Intuitively, this trend makes sense. If the economy is weakening, stocks should be declining and the incumbent party will likely suffer. Moreover, should it look like a new party is to take control of the White House, the change in control could add uncertainty to investors until the new President gets his or her rhythm."

"In fact, we have found that 'open' election years, a year in which no incumbent is up for re-election have been tougher for stocks than presidential reelection and non-presidential election years. Interestingly, stocks have rallied in the past two (and rare) instances when a political party has received a 3rd term."

"The S&P 500 increased 30 and 27% respectively in the year after Harry Truman won in 1948 and George H.W. Bush won in 1988. Sometimes the devil you know is better than the devil you don't know."

27 PHOTOS Women who support Donald Trump See Gallery Women who support Donald Trump Supporters of Republican U.S. presidential nominee Donald Trump recite the U.S. Pledge of Allegiance at a campaign rally in Panama City, Florida, U.S., October 11, 2016. REUTERS/Mike Segar Supporters of Republican U.S. presidential nominee Donald Trump attend a campaign rally in Ocala, Florida, U.S., October 12, 2016. REUTERS/Mike Segar A woman chants "Lock her up" in reference to Democratic presidential nominee Hillary Clinton as she attends a campaign rally for Republican presidential nominee Donald Trump in Miami, Florida, U.S., September 16, 2016. REUTERS/Mike Segar Republican U.S. presidential nominee Donald Trump holds up signs at the end of a campaign rally in Lakeland, Florida, U.S., October 12, 2016. REUTERS/Mike Segar Supporters of Republican U.S. presidential nominee Donald Trump attend a campaign rally in Panama City, Florida, U.S., October 11, 2016. REUTERS/Mike Segar Supporters of U.S. Republican presidential nominee Donald Trump cheer at a campaign rally, in Prescott Valley, Arizona, U.S., October 4, 2016. REUTERS/Mike Segar Supporters start to get excited before Republican presidential nominee Donald Trump takes the stage for a rally in Kenansville, North Carolina, U.S. September 20, 2016. REUTERS/Jonathan Ernst A woman looks on as Republican presidential nominee Donald Trump speaks at a campaign rally in Miami, Florida, U.S., September 16, 2016. REUTERS/Mike Segar A supporter of Republican U.S. presidential nominee Donald Trump holds a Trump doll as she listens to Trump speak at a campaign rally in Ambridge, Pennsylvania, October 10, 2016. REUTERS/Mike Segar Supporters rally with Republican presidential nominee Donald Trump in Bedford, New Hampshire, U.S. September 29, 2016. REUTERS/Jonathan Ernst Supporters of U.S. Republican presidential nominee Donald Trump look on as he speaks at a campaign rally in Pueblo, Colorado, U.S., October 3, 2016. REUTERS/Mike Segar Supporters of Republican presidential nominee Donald Trump cheer at a campaign rally in Manheim, Pennsylvania, U.S., October 1, 2016. REUTERS/Mike Segar A woman wearing a Muslim headscarf walks past people holding U.S. Republican presidential nominee Donald Trump signs before the start of the annual Muslim Day Parade in the Manhattan borough of New York City, September 25, 2016. REUTERS/Stephanie Keith A woman covers a child's ears as Republican presidential nominee Donald Trump speaks on stage during a campaign rally in Fredericksburg, Virginia, U.S., August 20, 2016. REUTERS/Carlo Allegri Audience member Robin Roy (C) reacts as U.S. Republican presidential candidate Donald Trump greets her at a campaign rally in Lowell, Massachusetts January 4, 2016. REUTERS/Brian Snyder TPX IMAGES OF THE DAY Republican presidential nominee Donald Trump supporters drive past the U.S. Democratic presidential candidate Hillary Clinton motorcade as she leaves a fundraiser event in Orlando, U.S. September 21, 2016. REUTERS/Carlos Barria A supporter of U.S. Republican presidential candidate Donald Trump attends a campaign event in Springfield, Illinois, United States, November 9, 2015. REUTERS/Jim Young Supporters of U.S. Republican presidential candidate Donald Trump attend a campaign rally in Council Bluffs, Iowa, January 31, 2016. REUTERS/Scott Morgan Republican presidential nominee Donald Trump is hugged by supporter Tracy McCullough during a campaign stop at the Boulevard Diner in Dundalk, Maryland, U.S., September 12, 2016. REUTERS/Mike Segar TPX IMAGES OF THE DAY Tracy McCullough (R), a supporter of Republican presidential nominee Donald Trump, cries after she had the chance to hug Trump during a campaign stop at the Boulevard Diner in Dundalk, Maryland, U.S., September 12, 2016. REUTERS/Mike Segar A woman smiles after getting an autograph by U.S. Republican presidential candidate and businessman Donald Trump on her hat after he spoke at a campaign rally South Point Resort and Casino in Las Vegas, Nevada January 21, 2016. REUTERS/David Becker A woman yells "Kill the media" before Republican presidential nominee Donald Trump speaks during a campaign rally in Austin, Texas, U.S., August 23, 2016. REUTERS/Carlo Allegri A supporter of Republican presidential nominee Donald Trump uses her phone to photograph Trump during a campaign stop at the Canfield County Fair in Canfield, Ohio, U.S., September 5, 2016. REUTERS/Mike Segar A woman poses for a selfie with supporters of Republican presidential nominee Donald Trump dressed in costume prison uniforms as Bill and Hillary Clinton sing the U.S. national anthem before a campaign rally in Austin, Texas, U.S., August 23, 2016. REUTERS/Carlo Allegri A woman in a U.S. flag themed dress poses for photos as Republican presidential nominee Donald Trump speaks on stage during a campaign rally in Fredericksburg, Virginia, U.S., August 20, 2016. REUTERS/Carlo Allegri A woman works her way to the front of the crowd as Republican U.S. presidential candidate Donald Trump signs autographs after addressing a Trump for President campaign rally in Macon, Georgia November 30, 2015. REUTERS/Christopher Aluka Berry Charmaine Adamo takes a selfie with friends and a Trump mascot "Trumpie" before a rally by U.S. Republican presidential candidate Donald Trump at the Myrtle Beach Convention Center in Myrtle Beach, South Carolina, November 24, 2015. REUTERS/Randall Hill TPX IMAGES OF THE DAY Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

Other unconventional indicators have also indicated a Trump win on November 8.

An artificial intelligence system that has correctly predicted the past three presidential elections as well as the Democratic and Republican primaries said Trump will likely win, and a professor who has accurately predicted the outcome of every presidential election since 1984 came to the same conclusion last month based on a model he developed that uses a series of true/false statements to determine who is best positioned to take the White House.

And after the FBI announced that it's reopening its investigation related to Clinton's use of a private email server during her time at the State Department, the polls started tightening, putting Trump within striking distance of Clinton.

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