Real Clear Politics compiles a running average of the polls that come in for 2016, just as it did in 2008.

We've done this before, but thought an update was warranted. So: Here's how Hillary Clinton's lead is faring in this year's average vs. how it evolved in 2008.

About five months before the New Hampshire primary (which was a month earlier in 2008), Clinton’s lead was at about 15. This cycle, that was a big plunge; in 2008, it was pretty much where she’d been. Then the lead in each cycle grew a bit, putting her back up into the 20-point range with about three months out. Then, quick drops, usually after voting happened. In 2008, the giant plunge came right after Iowa.

On Feb. 20, 2008, Clinton was already trailing Sen. Barack Obama, as she would permanently. This week, we saw a major poll for the first time putting Bernie Sanders in the lead.

The good news for Clinton is that she has got a big win on the docket next week: South Carolina, which she should win handily. In 2008, she and Obama traded big wins, back and forth, but she got beaten as he ran up the margins on delegates. Thanks to what happened with that yellow line in 2008, Clinton should know what to watch out for.

Read more:
What to watch in today’s Nevada and South Carolina votes
Sanders supporters are passionate — but maybe not as much as you think

The Fix's Aaron Blake explains why Bernie Sanders needs to make up ground with black voters in the Nevada caucuses and the South Carolina primary. (Peter Stevenson/The Washington Post)