So far, four Democratic presidential candidates — Eric Swalwell, John Hickenlooper, Seth Moulton and Jay Inslee — have dropped out of the 2020 race. What is amazing is that the number is only four, and that 21 Democrats are still running.

Of the 21 still in the race, 14 are polling below 2.0 percent in the RealClearPolitics average of national polls. Andrew Yang is at 1.8 percent; Tulsi Gabbard is at 1.4 percent; Julian Castro, 1.4 percent; Amy Klobuchar, 1.2 percent; Tom Steyer, 0.8 percent; Steve Bullock, 0.8 percent; Marianne Williamson, 0.8 percent; Kirsten Gillibrand, 0.6 percent; John Delaney, 0.6 percent; Tim Ryan, 0.6 percent; Bill de Blasio, 0.4 percent; Joe Sestak, 0.3 percent; and Michael Bennet and Wayne Messam are somewhere below that.

Why are they still in the race? Each has his or her reasons, but perhaps some are cherishing the hope, however far-fetched, that they might become the next Mike Huckabee or Rick Santorum.

Looking at the five competitive primary races of the last dozen years — Democrats in 2008 and 2016 and Republicans in 2008, 2012 and 2016 — only two candidates have come out of nowhere to lead the race. In the 2008 GOP contest, it was Huckabee, and in the 2012 Republican race it was Santorum. Both ultimately finished second.

In late August 2007, Huckabee, the former governor of Arkansas, stood at 3.0 percent in the RealClearPolitics average. He climbed slowly, hitting 10 percent in December 2007, and then shot into contention, leading the race for a few days in January 2008. John McCain then took the lead and eventually won the nomination.

Still, Huckabee had climbed from nowhere to somewhere.

Four years later, in late August 2011, Santorum, the former Pennsylvania senator, was polling at 2.1 percent. He was at 4.0 percent when January 2012 arrived, but then zoomed up the polls to lead the race at 30 percent for a while in February. By March 1, though, he was out of the lead for good when Romney pulled ahead for the nomination.

Still, Santorum had climbed from nowhere to somewhere.

Of course, it wasn’t enough; it should be noted that neither Huckabee nor Santorum won the nomination, nor was either picked for the vice-presidential slot, nor did either have any significant advantage when they ran again later on.

Nevertheless, there were benefits. After their runs, Huckabee and Santorum had far greater visibility, which translated into TV contracts, speaking fees and more opportunities. Each man was in a better place after running than before.

That could explain why some of the seemingly hopeless cases stay in the Democratic race. They don’t have a chance of actually winning the nomination. There is an overwhelming likelihood the eventual nominee will come from one of the seven Democrats currently above 2.0 percent in the RealClearPolitics average: Joe Biden at 28.8 percent; Bernie Sanders at 16.0 percent; Elizabeth Warren 15.4 percent; Kamala Harris 7.4 percent; Pete Buttigieg 5.0 percent; Beto O'Rourke 3.0 percent; and Cory Booker 2.2 percent.

But for those at the bottom, there is still the hope of hitting it big, or sorta big. And much recent primary history suggests that there will indeed be major changes in the race.

For example, in late August 2007 in the Democratic contest, Hillary Clinton held a lead of more than 15 points over Barack Obama, 37.8 percent to 22.2 percent in the RealClearPolitics average. Clinton stayed in the lead from August through January 2008, until Obama pulled ahead in February. He never gave up the lead again and went on to win the nomination and the White House.

The warnings for today’s front-runner, Joe Biden, are obvious.

On the Republican side in the 2008 race, in late August 2007, the leader was Rudy Giuliani, who held a 10-point advantage over Fred Thompson, 27.7 percent to 17.0 percent. Giuliani’s lead lasted until January 2008, after which he fell steadily.

It’s another obvious warning for Biden.

But what about the rest of today’s Democratic field — not the cellar-dwellers, but the ones who might have a real chance? There are lessons for them, too, especially in the 2012 Republican race. Like today’s race, it was fairly stable, with Romney in the lead, until August 2011. Then all hell broke loose.

First, Rick Perry took the lead. Then Romney took it back. Then Herman Cain surged into first place. Then Romney rose again, briefly. Then Newt Gingrich took the lead. Then Romney again. Then Gingrich again. Then Romney again. Then Santorum. And finally, Romney.

Everybody had a chance, or at least thought he had a chance, at some point in the race.

So there are reasons why today’s Democratic field remains so large. The candidates in the top tiers are quite reasonably expecting some sort of re-sorting in coming months. And the candidates at the very bottom are looking to improve their lot before the inevitable surrender. It could take quite a while before many are convinced to give up.

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