The performance of the S&P 500 Index has signaled the outcome of every presidential election since 1984, according to an analysis by Strategas Research Partners LLC.

A gain in the benchmark for American equity in the three months prior to the vote has seen the incumbent party win 86 percent of the time since 1928. Right now, the benchmark gauge is down 3.6 percent since Aug. 8 with just a week until the vote, a fact that in isolation augurs well for Donald Trump.