The stock market's election year performance between July 31 and Oct. 31 has often accurately predicted the next president — and this year it's pointing to a victory by Donald Trump, if history is a guide. Sam Stovall, chief investment strategist at CFRA, says the market's decline this fall has been a bad omen for the incumbent party and Hillary Clinton, who still holds a six point national lead in a new poll. The S&P 500 is down 2.2 percent since its close of 2,173 on July 29, a Friday and the last trading day of July. "Going back to World War II, the S&P 500 performance between July 31 and Oct. 31 has accurately predicted a challenger victory 86 percent of the time when the stock market performance has been negative," he said. The one time in eight that the incumbent party won with a negative…