Stocks fell last week after FBI director James Comey told Congress the agency had discovered emails 'pertinent' to Clinton's server investigation

Wall Street seems to be calling a win for Donald Trump.

The S&P 500 has fallen 2.2 per cent over the last three months. Historically, a decline in stock prices has forecast a win for the challenger candidate.

The index has called it correctly in seven of the eight past elections, so the fall in stock prices is a bad omen for Hillary Clinton, Sam Stovall, chief investment strategist at CFRA, told CNBC.

'Going back to World War II, the S&P 500 performance between July 31 and October 31 has accurately predicted a challenger victory 86 percent of the time when the stock market performance has been negative,' he said.

This July the final day of Wall Street trading was Friday July 29.

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Trump win? The S&P 500 has fallen 2.2 per cent over the last three months. Historically, a decline in stock prices has forecast a win for the challenger candidate

With exactly a week to go before the election, polls have shown contrasting pictures of the state of affairs since the FBI told Congress it was investigating emails relating to its probe of Clinton's private server - which it found on the laptop of Anthony Weiner, estranged husband of longtime Clinton aide Huma Abedin.

Stovall said that of the previous eight elections, the only time the incumbent party won during a declining stock market was fifty years ago, when President Dwight D. Eisenhower fended off Adlai Stevenson.

Stovall noted that the drop could be on concerns about a Democratic sweep of the House of Representatives as well as the presidency, although those have fallen away since FBI director James Comey said his agency were looking at the email trove.

The emails emerged through the investigation of Weiner for sexting with a 15-year-old girl.

Stocks took a hammering on Friday after the news broke. The S&P 500 fell 0.5 per cent, as did the Nasdaq composite Index. One analyst told AFP that the market reaction 'is essentially a reflection of the possibility that Trump is going to win.'

The S&P 500 has declined 2.2 per cent since Friday July 29, the final trading day of the month, and Halloween - forecasting a win for the challenger party, according to one expert. The last time the market called it wrong was 50 years ago

A trader at the New York stock exchange on Halloween

Also on Tuesday Trump pushed past Clinton for the first time in the ABC News / Washington Post daily tracking poll.

But a NBC News/Survey Monkey poll conducted since the email news showed Clinton maintaining a six-point lead.

Nate Silver's FiveThirtyEight blog has calculated a fall in the probability of a Clinton presidency: The Democrat has a 74.1 likelihood of success, against a 82.2 per cent on Oct. 27, the day before the revelations.

Even before the FBI's bombshell, however, the S&P was still on track for a decline over the three-month period, albeit to a lesser extent. The market closed down 1.9 per cent on Oct. 27.