After Quinnipiac University released the results of a new national poll of the 2020 Democratic primary race, a brief snowball effect occurred. That poll, giving Sen. Elizabeth Warren (D-Mass.) a slight within-the-margin-of-error lead over former vice president Joe Biden, shifted her average in national polls up just a bit and Biden’s down just a bit.

The result? For the first time, RealClearPolitics gives Warren the lead in the Democratic contest.

Warren’s current position is a function of a slow improvement over the spring and summer — and a recent surge that’s come in part at the expense of Sen. Bernie Sanders (I-Vt.). Biden, meanwhile, has been hovering in the upper 20s since July, after spiking following his campaign announcement.

So now what? What happens next? Will Warren hold on? Will her surge similarly fade? What does history tell us? Happily, we have similar RealClearPolitics averages for each contest since 2008, allowing us to compare past surges with Warren’s.

Lead changes happen less frequently than you might expect. In the five contested nomination fights since 2008 (excluding the Democrats in 2012), there have been only 16 lead changes within the last 500 days of the race. More than half of those came in the 2012 Republican field alone. In the Democratic contest in 2016, there wasn’t a lead change at all.

About half the time, the person who surges into the lead loses it within three weeks. The other half of the time, the person holds the lead for at least that period. In four cases, the candidates who took over the lead never relinquished the lead, going on to win the party’s nomination. (The fifth, of course, was Hillary Clinton in 2016.)

Here’s what Warren’s recent surge into the lead looks like relative to next year’s general election. The graph shows the three weeks leading up to her taking the lead and the days since. (The rest of the graphs below also show the three weeks following for each candidate.)

In 2008, the lead changes for both the Democrats and the Republicans happened closer to the election — after voting had started, in fact. Former Arkansas governor Mike Huckabee took the lead briefly after winning the Iowa Republican caucuses, soon giving it up (permanently) to then-Sen. John McCain. Then-Sen. Barack Obama took the lead from Clinton after voting began, holding it through the rest of the primaries.

In 2016, there were three shifts on the Republican side, despite the crowded field. In July 2015, Donald Trump took the lead from former Florida governor Jeb Bush. In early November, he gave it up briefly to Ben Carson — but only briefly.

Warren’s recent polling looks more like Trump’s July surge or McCain’s in January 2008 than Huckabee’s or Carson’s. That would seem to be a good sign for her.

But then there’s 2012.

That was a bizarre primary. There were a lot of candidates by historical standards, but relatively few by the standards of either the 2016 Republican or 2020 Democratic fields. Over the course of the last 450 days, the Republican lead changed nine times.

There was former Texas governor Rick Perry, who surged into the lead before stumbling in a debate. Businessman Herman Cain saw a surge that was slightly faster than Warren’s, and which flatlined soon after he took the lead. Former Georgia congressman Newt Gingrich took the lead twice, giving it up to Mitt Romney both times. After former Pennsylvania senator Rick Santorum won the Iowa caucuses, he briefly surged into the national polling lead before giving it up to Romney.

Biden would certainly like to think that he’s Romney in this scenario, briefly giving up a lead that he’ll finish with. This may be the only scenario in which Warren would like to emulate Trump, seizing a lead that she holds essentially uncontested through the rest of the primaries.

It’s hard to tell who’s right. By this point in 2012, there had been only two lead changes; perhaps we have half a dozen more waiting for us in 2020. By this point in 2016, there had been just one within the final 500 days, and it basically stuck.

One of the more instructive past surges might be the Perry one. As we’ve noted before, Perry was leading the Republican field in October 2011 but was also seen as the person with the best chance at beating Obama in the general election.

Oops. That also changed.