At this point in the 2012 election, President Barack Obama lead rival Mitt Romney by just 0.7 of a point, but went on to win by 5 million votes and 332 of the 538 electoral votes. In 2004, Democrat John Kerry was within 1.6 points of then-President George W. Bush, who won with a 3 million vote lead and 286 of the 538 electoral votes.

This year's poll gap is more like the 2008 race between Sen. John McCain and Obama, who was up by 5.5 percentage points 28 days before the election. Obama went on to win by nearly 10 million votes and outpolled McCain's electoral vote count by more than 2 to 1.

As in past races, poll numbers tend to swing frequently early in a presidential race. But results from the last three presidential campaigns also show that that voter preferences tend to solidify once the election is less than 30 days away.

Trump's numbers surged after the Republican convention in July, briefly overtaking Clinton by about a percentage point. But the surge faded as the Democrats took the stage with their convention a week later. The Trump campaign narrowed Clinton's lead back to within the margin of error in late September. Both candidates picked up support earlier this month as undecided voters began committing themselves.

But the poll margin began widening on Oct. 3, days before the publication of Trump's taped comments about groping women.

Since then, the gap in poll results has widened markedly.