Donald Trump is president of the United States in large part because he quickly established a core base of support in 2015, which helped power him through the Republican primaries and to the nomination. After he announced his candidacy in June of that year, he polled relatively poorly — until a national debate over his comments about immigrants from Mexico erupted. That controversy, framed in the media as a natural backlash against an unlikely candidate, drew enormous attention to his hard-line views on immigration and led to a spike in support.

Trump was saying things that were common in conservative media but uncommon among Republicans seeking elected office, and he was rewarded with a loyal base that has largely stuck with him since. In a 17-candidate field, that core made him an instant front-runner as support for other candidates tended to ebb and surge. He won the nomination, his party coalesced around him and here we are.

As it stands now, the winnowed 2020 Democratic field is still about as robust as the 2016 Republican field was at its peak. While there are a dozen candidates running, there is a top tier of four candidates outpacing the rest of the field and seems positioned to do so for some time moving forward.

Within that top tier, though, there are two types of candidates. Former vice president Joe Biden and Sen. Bernie Sanders (I-Vt.) come into the first voting states with core bases of support that have remained largely intact for the past six months. Sen. Elizabeth Warren (D-Mass.), on the other hand, has seen her support rise and wane. Former South Bend, Ind., mayor Pete Buttigieg (D) has seen a smaller surge and fade.

Put visually, the distinction looks like this. Biden and Sanders have only seen their national poll numbers move within a six-point range. Buttigieg’s range is slightly wider, about eight points, but with a pronounced surge. Warren, by contrast, has seen her national polling spread across 13 points — as high as 27 points (briefly matching Biden) and as low as 14.

In the Democratic contest, those core bases of support have a different significance. While many of the 2016 Republican contests had delegate allocations that were winner-take-all (meaning that Trump’s plurality of the vote yielded him a disproportionate number of delegates), the Democratic contests award delegates proportionally — once a candidate hits a baseline of 15 percent support in a state. If Biden and Sanders hold bases that are greater than 15 percent of the voters, it positions them well to earn delegates in each state.

But it’s not quite that easy. All states are not created equal and, in some states, those bases of support aren’t what they might be elsewhere.

Consider Iowa, for example. Iowa is, as you’ve no doubt heard by now, mostly white. A party that is 59 percent white holds its first contest in a state that is 85 percent white. If you’re a candidate whose base is heavily nonwhite, then you’re at a disadvantage. If, for example, you’re Joe Biden.

If, by contrast, you’re an ebb-and-surge candidate who does better with white voters — like Buttigieg — Iowa might be expected to be disproportionately strong for your candidacy.

Using the same visualization as above, we see a much muddier scene in Iowa with no clear front-runner. Sanders polls about as well in Iowa as he does nationally (20.3 percent vs. 18.8 percent in RealClearPolitics’ averages). Biden fares much worse in Iowa (20.7 vs. 27.4) and Buttigieg much better (18.7 vs. 7.8).

Those demographic differences weren’t as significant a factor in 2016 for the simple reason that the Republican Party is much more homogeneously white. You’ll notice, too, that both Warren and Sanders have seen more stable support in Iowa than have Buttigieg and Biden. That, too, undermines the idea that having a core base of support in the Democratic 2020 contest is as certain a factor in outcomes as it was four years ago for the opposition party.

That said, this idea that Sanders and Biden have a steadier base of national support is reinforced by recent polling. Quinnipiac University polling released Monday indicates that supporters of both Biden and Sanders show firmer support for their candidates than to Democrats overall (41 and 42 percent respectively, relative to 35 percent overall). Warren supporters, on the other hand, are significantly less likely to say their support is firm, with only 28 percent saying they won’t change their minds before voting.

Again, though, contrast that with Iowa. In the state that has been the target of the most electioneering so far, more than half of the supporters of each of the top four candidates say their support is either firm or that there’s a low chance their opinions will change. Warren’s support is softer here, too, but she fares better than she does in the national Quinnipiac poll.

Obviously, candidates would rather be Biden — leading nationally and with a steady base of support — than, say, Buttigieg. But while Trump’s nomination (and presidency) was in large part a function of his core base of support, Biden’s and Sander’s bases don’t provide an advantage in precisely the same way.

And notice the second column in that Quinnipiac poll, above. Even for Biden and Sanders, more than half of their voters say they could change their minds.

In a Quinnipiac poll conducted in December 2015, about a third of Republican primary supporters of Sen. Ted Cruz (R-Tex.) said they might change their minds. Sixty-three percent of Trump backers said they wouldn’t — and it’s safe to assume that they didn’t.