Since 1944, the outcome of the U.S. presidential election could be predicted with a high level of confidence by looking at the performance of the S&P 500 over the three months preceding polling day.

If the S&P 500 rose from July 31 to Oct. 31, the party controlling the White House won the election 82% of the time, according to calculations by S&P Dow Jones Indices. If the S&P 500 declined during that period, the opposition party won 88% of the time.

This year, the benchmark stock index was almost unchanged during that three-month period, falling just 0.03%.

According to poll aggregator FiveThirtyEight, Democratic presidential nominee Joe Biden leads President Donald Trump by 8.4 percentage points nationally, but the race is tighter in the most important swing states.

The first polls opened at 5 a.m. ET in Vermont, while almost 100 million people have already cast their ballots in early voting, suggesting the country is set for its highest turnout in a century.

High numbers of mail-in ballots mean that a result may not be announced for days, while the Trump campaign has promised to challenge the inclusion of ballots received after polling day.

The S&P 500 was 2.5% higher, excluding dividends, this year through yesterday, even after slumping 8.4% in February and 12.5% in March as the fallout from measures to contain the coronavirus pandemic gripped markets. It rebounded 12.7% in April and reached record highs in August, before declining in September and October.

In presidential election years since 1945, the index has risen 78% of the time for an average gain of 6.3%, S&P Dow Jones Indices calculations show. That compares with gains in 71% of all years since 1945, including election and non-election years, for an average 8.9% increase.

S&P Dow Jones Indices and S&P Global Market Intelligence are owned by S&P Global Inc.