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.