An average of recent national polls shows Clinton with about 46 percent support to 39 percent for Trump and 6.4 percent for Liberatarian candidate Gary Johnson.
Trump predicts an electoral surprise, or "new Brexit," on November 8.
But as he's lost ground, the New York billionaire has attacked the election process itself, complaining of a massive conspiracy to rig the elections against him.
"The press has created a rigged system and poisoned the minds of the voters," he said Tuesday.
"Either we win this election or we lose the country.