History says Trump can't come back

The polls are coming fast and furious right now, and they're increasingly bad for Donald Trump. In fact, we can now say this: Were Trump to actually win, it would represent the biggest late comeback in the history of presidential polls.

Monmouth University is the most recent to show a big swing in Hillary Clinton's favour. Its just-released national survey has her leading Trump by 12 points, 50-38. That 12-point margin is the biggest so far this month. A new NBC-Wall Street Journal poll over the weekend also had Clinton ahead by double digits - 11 points - and other recent high-quality polls have shown her up 9 points (NBC-WSJ last week), 8 points (George Washington University), 7 points (Fox News) and 4 points (Washington Post-ABC News).

According to HuffPost Pollster, Clinton's average lead in these high-quality polls is now 8.9 points.

At this point in the cycle, no candidate who has seen such big deficits in any poll - much less the average of all polls - has recovered to win. The only poll of the six mentioned above that is even within the margin is the Post-ABC poll.

Now, to the caveats:

First off, there are lots of polls today, but that wasn't always the case. Before the turn of the century, we're largely talking about only Gallup polls, and even those were infrequent the further back you go. If there were more polls conducted way back when, perhaps we would have seen a late double-digit deficit or two overcome.

Secondly, we're only talking about 20 elections for which we have Gallup data, from 1936 to 2012. So the fact that a comeback this big hasn't occurred over that span doesn't mean it's impossible - just that it hasn't happened in 20 elections.

And third, we have seen big late swings, but they were generally a candidate who trailed by a modest amount going on to win by a comfortable margin - not a candidate who trailed big coming back to win.

Here's the three most recent examples of candidates overcoming late deficits to win.

2000 - George W. Bush

A couple of isolated polls in mid-September and early October showed Bush trailing Al Gore by 14 points (Newsweek) and 11 points (Gallup), respectively. But those were basically the only polls that showed such a lopsided race. By this point in the cycle, Bush had asserted a lead in most polls.

1980 - Ronald Reagan

Gallup showed Reagan trailing Jimmy Carter by six points in mid-October, 45-39. That 6-point deficit is less than all but one of Trump's deficits right now.

1948 - Harry Truman

Thomas Dewey led Harry Truman by 5 points, 50-45, in mid-October Gallup polling and wound up losing the popular vote by nearly 5 points.

Presidential candidates have swung races by double digits between their worst October poll and Election Day. But that's not from the average of polling. FiveThirtyEight's Harry Enten compared polling averages to the final results. He found the biggest shift was actually in 1992, when mid-October polling averaged an 8.5-point bigger Bill Clinton lead than the final result showed. But that's still not as big as Trump's 8.9-point deficit, and it wasn't enough to swing that race.