Around this time in 2016, I wrote a piece arguing that, while some pundits were speculating about a contested GOP convention, in fact then-candidate Donald Trump was on the brink of a historically dominant primary run. Regardless of whether he won Iowa, he was highly likely to win New Hampshire, and after that South Carolina. If he started with an Iowa win, he might run the table; if he lost Iowa to Ted Cruz, he was still by far the favorite to be the nominee. "A Trump nomination would be unprecedented," I concluded. "But an upset victory by any of his opponents would, in many ways, be even more so."

Trump, of course, wound up losing Iowa, but nonetheless went on to win New Hampshire, South Carolina, and the nomination, on the way to his improbable general election victory.

So how about this year? As with the Republicans in 2016, the race has been quite stable at the top. Though Elizabeth Warren briefly took a lead in some national polls in October, and Bernie Sanders has led in a couple of more recent polls, Joe Biden has held the first place in the polling average the entire time he's been a candidate. Although punditry (and the party leadership) has been lukewarm on him from the start, he has been and remains the frontrunner. Does that mean he's also poised for something close to a sweep?

It's still possible. Though Sanders has been surging recently in both national and early-state polls, Biden led in Iowa as recently as one week ago, and his lead remains formidable in South Carolina. If he wins both states decisively, and especially if he also wins Nevada, he'll be in a strong position come Super Tuesday, no matter how well Sanders does in New Hampshire (where his lead appears to be widening). But the opposite is also possible. Biden could lose all three of the first contests — indeed, he could fail to even come in second — and his victory in South Carolina is not guaranteed.

What happens then? Contrary to the Republican situation in 2016, if Biden doesn't dominate the early caucuses and primaries, a contested convention becomes a very real possibility. Indeed, I think it may be considerably more likely than statistical models, like Nate Silver's at FiveThirtyEight, suggest.

Why the difference? It's all about Bernie and the Billionaires.

Statistical models based on past elections make certain reasonable assumptions about candidate and voter behavior that may be getting less applicable. For one thing, they assume that candidates will drop out when they no longer have a realistic path to victory. But that's only a valid assumption if a candidate has limited resources, and if the candidate's paramount interest is unifying the party. Those assumptions may be much less valid when considering candidates with independent resources and an agenda independent of the party's.

Even before his recent rise in the polls, Bernie Sanders had powerful incentives to fight all the way to the convention — and his prodigious small-dollar fundraising assured that he would have the resources to do so. If he could hold out to be the last opponent of the likely nominee, he could become the vehicle for any lingering dissatisfaction with the party or the process, as he was in 2016. And if multiple candidates remained viable until late in the game, his continued presence would make it harder for any candidate to accumulate a majority of delegates — and in a contested convention, his delegates would be valuable bargaining chips to trade for his causes.

The only scenario where it would have made sense for him to leave early would be to consolidate left-wing support around another, more viable progressive candidate, presumably Elizabeth Warren. But with Sanders' position in the polls strengthening, that script is flipped, and it is Warren's campaign that risks coming under pressure to fold if she performs poorly in the early states. (In a recent poll from Morning Consult, 37 percent of Warren supporters chose Sanders as their second choice.) Whether she would give in to such pressure until it was financially unavoidable, though, is rather more doubtful after their recent contretemps.

In a two candidate race between Sanders and Biden, would Biden be favored to win, if only because of his much stronger support from party loyalists? Could 2020 be a repeat of 2016 in other words? It’s a good question. Biden is a much weaker front-runner than Hillary Clinton was, and Sanders a much more formidable challenger the second time around. But the question might be academic. If Biden stumbles, it's unlikely to be a two-candidate race, regardless of what Warren (or Buttigieg) does, because of the billionaires: Tom Steyer and Michael Bloomberg.

Once upon a time, Steyer and Bloomberg would not have been considered plausible candidates for the Democratic nomination. But now they are testing whether Trump's rise was a fluke, or whether he was demonstrating new possibilities for unconventional candidates with vast personal resources. Even as once promising contenders like Kamala Harris and Cory Booker have fallen by the wayside, the two billionaires have turned themselves into meaningful factors. Steyer earned and held his spot on a shrinking debate stage, and he has polled in the double digits in some recent polls of both Nevada and South Carolina. Bloomberg promises to be a considerably more significant force down the road. In spite of skipping the debates and ignoring the first four contests, Bloomberg has pulled into fourth place in national polls, passing Buttigieg, and he's made a massive investment in the Super Tuesday contests.

If Biden limps into March 3, then, there will likely be a substantial number of Democrats — particularly those who are reluctant to feel the Bern — looking for a new home. And Bloomberg will be waiting on their doorstep, jangling the keys. Indeed, even if another candidate dramatically outperforms expectations — if Warren wins Iowa and Nevada, say — it is likely that Bloomberg will remain a factor, just as Sanders will. The reason: In recent head-to-head polling against Trump, in both national polls and swing states, Bloomberg polls about as well as Biden does. That's not a claim either Warren or Buttigieg can make.

Unless Biden is the clear front-runner, then, it is likely that there will be at least three and possibly as many as five or six viable candidates splitting the delegate haul on Super Tuesday.

Why could that lead to a contested convention? It didn't in 1988, which followed a similar pattern. That year, Richard Gephardt won Iowa, Michael Dukakis won New Hampshire, and Super Tuesday was a split decision, with Jesse Jackson winning states with large African-American populations, like Alabama, Georgia, and Louisiana, Al Gore winning states of the border South similar to his home state of Tennessee, like Kentucky, Arkansas and Oklahoma, and Michael Dukakis winning the biggest prizes of Florida and Texas along with states outside the South like Rhode Island and Washington. Dukakis nonetheless managed to consolidate a majority of support down the road. Why shouldn't the delegate leader after Super Tuesday 2020 fare similarly? For that matter, why couldn’t Sanders be the one to replicate Trump’s performance in 2016? If he wins Iowa, New Hampshire and Nevada, and is the delegate leader after Super Tuesday, why couldn’t he mostly run the table thereafter?

The answer is that under the Democrats' new rules, that could prove mathematically challenging. By March 3, nearly 40 percent of the pledged delegates will have been chosen. Those delegates will be allocated proportionately to the vote in each state and district, among candidates who earned at least 15 percent of the statewide vote, and at least 15 percent within each district. If as many as five candidates have managed to clear the threshold in several different states, it's not implausible that no candidate would have won more than a third of those delegates.

In that eventuality, the delegate leader would need to earn upwards of 60 percent of the remaining delegates to get a majority. That's a daunting number: It would require the leader to win very nearly every remaining state, including Bloomberg's New York, Biden's Pennsylvania, and states like Michigan and Wisconsin that have proved fertile ground for Sanders in the past. Finally, with superdelegates no longer eligible to vote on the first ballot, if there's no majority, then we've got a contested convention.

It’s unclear whether establishment Republicans in 2016 could have closed ranks to stop Trump; there may have been no pro-establishment majority at all, but rather an anti-establishment majority divided between Cruz and Trump. But under the Democrats’ rules, they wouldn’t have needed to; a divided field of candidates all earning delegates would have denied any candidate a majority. The same dynamic that could keep a wounded establishment front-runner like Biden from limping over the finish line would also hamstring an insurgent like Sanders even if he broke through.

Such an outcome is still considerably less-likely than not. Among other things, most Democrats are surely eager to have a nominee with the clear backing of the rank and file; the last thing they want is a nominee that some faction can complain won in a smoke-filled room rather than in a fair and open fight.

But those same Democrats have limited ability to coordinate towards that outcome. If a leader doesn't emerge fairly early who they are willing to rally around, whatever their reservations, the party may belatedly discover that failing to decide has dropped the decision more plainly in their laps than any time since 1968.

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