By the time March 2007 rolled around, the 2008 presidential contest seemed to have clear front-runners. On the Democratic side, then-Sen. Hillary Clinton (N.Y.) seemed well positioned to secure her party’s nomination. And on the Republican side, a former mayor of New York, Rudolph W. Giuliani, led in polls by double digits.

Neither Clinton nor Giuliani ended up far from national politics, of course, but neither earned a nomination that year. Clinton was passed in the stretch by Barack Obama, while Giuliani’s sort-of-campaign never really got started.

The point being that we are shortly going to enter a period in which poll after poll after poll will try to give us a sense of how the Democratic primary field is shaping up, and, invariably, some of those polls will be used to declare that one candidate or another has a near-insurmountable lead. In 2015, we sought to combat that by gently reminding people of who had led in prior presidential primary contests on a day-by-day basis. And in 2019, we’re doing the same thing.

Relative to the November election in each year, here’s who was leading in past races.

If you’re a social media type, you can also follow our Twitter account that will share the past front-runners every day. It’s at @LedPast. (The account we used in 2015 was apparently stolen somehow by a British company that does SEO marketing so, uh, sorry if you got weird British tech tweets in your feed for a while.)

Follow @LedPast on Twitter

All of this uses daily poll averages each year from RealClearPolitics. Earlier in the campaign, there are fewer polls and therefore more volatility.

You’ll notice something interesting, though, as the year progresses. Here are the periods from 650 to 250 days before the general election. By 250 days out in each of the five elections we’re looking at, the parties had identified their candidates. Notice, though, how dominant Clinton was on the Democratic side in both 2008 and 2016. There are fewer than 20 days in the two years when someone besides Clinton held a lead in the Democratic nominating contest: Obama, at the end of 2008.

The drop in Clinton’s advantage over the course of 2015 is still remarkable to consider — though the plunge near the Day 300 mark in 2008 (about when we’ll transition from 2019 to 2020) that followed her third-place finish in Iowa is also dramatic.

This year, with an expected glut of Democratic candidates and a lack of a clear favorite like Clinton, we might expect the Democratic polling to look more like 2012 or 2016 on the Republican side, a constant jockeying back and forth for the lead as the campaign progresses.

All the more reason, then, to remember how common such lead changes have been in recent years when a presidential contest doesn’t include Hillary Clinton. And, for that matter, how even when a contest did include such a clear front-runner, surprises can still happen.