Donald Trump holds a campaign event in Miami, Florida U.S. November 2, 2016. REUTERS/Carlo Allegri 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.

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."

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."

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.