After the release of the NBC-Wall Street Journal poll on Monday, RealClearPolitics updated its national polling average to reflect the shift in the presidential race. That new poll shows Hillary Clinton with a big lead over Donald Trump, some 14 points in the wake of the hot-mic videotape that's undercut Trump's support. In the absence of other polls showing a similar spread, though, the average shows a more modest 6-point gap between the two.
From a historical perspective, it's normal to see movement shortly after the first debate. In 2000, as you can see on the chart below, the red line shifted back toward George W. Bush after the first debate. In 2012, as the yellow line shows, the race shifted away from Barack Obama.
More important, though, notice that the 2016 line, the thick blue one, is steadily moving farther to the right. Clinton's lead now (or, actually, as of Sunday, the date of the completion of the NBC-Wall Street Journal poll) is higher than at the same point in any of the other cycles except 2008. That year, with 30 days to go, Barack Obama led John McCain by 5.9 points. Now, Clinton leads Trump by 5.8.
But notice, too, how similar the lines are from those two years. We can isolate them to make that comparison easier. For the last 30 days, Obama never trailed by more than 5 points.
This is a very bad sign for Trump, but not because the comparison will necessarily hold. It's a bad sign for him, as we've noted, because he needed the line to move to the left in the last stretch, not farther to the right.
What happened? Well, we can turn to 2008 to explain that, too. At about the 50-day mark in 2008, the global economy tanked, which helped propel Obama's victory. This year, there was a similar shock. It wasn't Wall Street that crashed, though. It was Trump.