Former Starbucks chief executive Howard Schultz is threatening to mount a candidacy that could upend the 2020 presidential race: a well-funded “centrist independent” bid that would seem to have the resources to get on the ballot in all 50 states. Schultz told “60 Minutes” in an interview that aired Sunday that he’s looking at it, and the fact that he did the interview suggests it’s quite likely.

But how much appetite is really there for such a campaign? And would Schultz — a longtime Democrat who clearly leans left — just turn into the “spoiler” for President Trump that Democrats immediately worried he would?

The nature of independent presidential campaigns and the current political paradigm make it difficult to predict anything with much certainty. We’ve rarely had someone put this question to the test, because independents and third-party candidates tend to be a) bad candidates and b) cash-poor. Our system is also built for two major parties. The practical effect is that independents and third-party candidates rarely compete at the presidential level and rarely win seats in Congress, unless they are running as the de facto nominee of one party or another (see: Bernie Sanders).

But we’ve also never had Trump as president. And the Democrats' 2020 field seems to be shifting quite a bit leftward. We’re also just a quarter-century from a presidential race in which another billionaire, centrist independent led in the polls for a time.

Schultz thinks there’s a market again.

“What we know, factually, is that over 40 percent of the electorate is either a registered independent or currently affiliates themselves as an independent,” Schultz said. “Because the American people are exhausted. Their trust has been broken. And they are looking for a better choice.”

The first thing we can say is that Schultz is about right — at least with his data point. Gallup polling shows that the percentage of people who self-identify as independent (39 percent) surpasses the percentage who identify as Democrats (34 percent) and Republicans (25 percent). This number has hovered around 40 percent the past few years and is up slightly from earlier this century.

As any political scientist will quickly point out, though, “independent” isn’t always truly independent, and most of these voters clearly favor one party over another. The same Gallup data show that 12 percent of voters — about one-third of all independents — don’t lean toward one party or another. Fully 88 percent of voters have a home base on one side or the other.

Many of them are completely reliable voters — especially on the right, where the desire to label oneself “independent” is stronger than it is on the left. This consistently leads to more Americans identifying with Democrats rather than Republicans, even though the two parties generally split the vote pretty evenly.

But just because they “lean” one way or another or identify with a party doesn’t necessarily mean they would never consider a candidate such as Schultz. Gallup polling also shows that a clear majority of Americans — 57 percent — say a third major party is needed.

Via Gallup (Screenshot/Gallup)

Polling earlier this decade showed a similar openness. A Rupe poll for the libertarian magazine Reason in 2011 found that 60 percent of Americans would consider a third-party or independent candidate and that 20 percent might.

There is little doubt that these polls oversell how much support such a candidate would take, though. If there’s one thing polling consistently shows, it’s that people like the idea of choices. What could be so bad about a third option? In the abstract, it sounds great.

In practice, it’s another matter entirely. And our system is essentially built to prevent these candidates from getting off the ground. It’s called Duverger’s Law, and the Monkey Cage explains it here:

Here’s how it works. First, when each district gets only one legislative seat (known as a single-member district, which we have in the United States) and, second, when the election’s winner takes that seat, then the system tends to have two dominant parties.

In such a system, all a party needs to win is more votes than the other side. That winner-takes-all nature of single-member districts encourages broad coalitions to form before elections. The odds of a party winning such elections are much higher if only two parties exist, enabling each side to work to bring as many people to its side as possible.

In the United States, that’s writ large in presidential races, because the Electoral College is itself a winner-take-all system: Within each state, the candidate who wins more votes takes all that state’s electoral votes. Even in Nebraska and Maine, where electoral votes are allocated by congressional districts, each individual district is winner-take-all.

This rallying effect is most likely to take place on the left, where there will be real concern about whether Schultz could take enough votes from the Democratic nominee to help Trump win reelection.

Immediately after his interview Sunday, liberals on social media warned him that he might as well be campaigning for Trump. And Trump seemed to practically beg Schultz to get in (under the guise of attacking him):

At the same time, it’s not guaranteed that Schultz would take more votes from the left than the right. Although his politics certainly lean that way, there could be more of a market among disaffected Republicans than Democrats (depending on whom their nominee is). Trump has locked up much of his base, but polling has shown cracks in it. There certainly seems to be more danger for Democrats here, given that they are favored to win the election at this point, but independent bids don’t tend to break down as neatly as we expect them to.

The name of the game for Schultz will be to reach viability — to rank high enough in polls to get a spot on the debate stage (generally 15 percent) and convince people that he wouldn’t be just a protest vote. Ross Perot cleared that threshold for a while and then buckled under the pressure when the scrutiny increased.

It’s highly unlikely that Schultz would win, but if 2016 showed us anything, it’s that unprecedented things can and do happen in presidential elections. We’re in uncharted territory. But Democrats will fight like hell to keep this a two-party race, and that means getting off the ground would be difficult for Schultz.