South Bend, Indiana, Mayor Pete Buttigieg has surged into a statistical tie with Sen. Elizabeth Warren for the lead in Iowa, a new survey suggests. If his rise continues, it would increase the chances that Democrats will head into Milwaukee next July without a presumptive nominee, triggering a contested convention.

It's true that pundits often get excited about the prospects of a contested convention, only to see one candidate consolidate support as the field narrows. But three factors are in play this year make the possibility of a contested convention more likely: 1) The massive Democratic field, 2) Democratic Party rules that allocate state delegates on a proportional basis, and 3) Under new rules, super delegates (who can still support whoever they want) won't be able to put a candidate over the top on the first ballot.

In 2016, though the Democratic nomination was competitive, it quickly became a two-person race, which made it easier for Hillary Clinton to win a majority of delegates. And though the Republican primary field was historically large, the party's allocation rules were different, allowing Donald Trump to rapidly amass delegates by edging out rivals in winner-take-all states.

For Democrats to avoid a contested convention (which I'm defining as no single candidate amassing a majority of delegates by the end of the primaries), they need for the field to narrow to essentially two main candidates relatively early on. This seemed to be a possibility when it was looking like Warren and Joe Biden were co-front-runners. But the latest Quinnipiac poll shows a four-way race in the first state, with Warren at 20%, Buttigieg at 19%, Sen. Bernie Sanders at 17%, and Biden (the national front-runner) at 15%.

A Buttigieg victory in Iowa would be particularly disruptive. A big reason is that for months, polls have consistently shown him in the low single digits among black voters, who play a critical role in later states. While it might be conceivable for a candidate to lose black voters and still win, no Democrat will be able to do as poorly as Buttigieg has been performing and have a path to the nomination. But if he wins Iowa, he'll have ample reason to justify staying in the race for a long time (and the cash to do it), which would allow him to vacuum up delegates in states and congressional districts with whiter electorates.

Warren has not polled particularly well among black voters either (typically in the low double digits), though she does do considerably better than Buttigieg. Her hope of the nomination would hinge on racking up early victories to provide her momentum going into the later contests and the hope that Biden sputters out of Iowa and New Hampshire and is weak by the time South Carolina comes around (where he still enjoys a comfortable lead due to his dominant position among black voters). Her strategy would be severely weakened by a Buttigieg victory in Iowa, but she would likely still be strong enough to hang around the race for awhile.

Sanders, meanwhile, is proving resilient. He's an ideological warrior with tons of cash and a loyal following. Typically, that type of candidate sticks around for a long time — and it will be especially true if he remains in the mid-to-high teens.

Keep in mind that the threshold for delegates tends to be set at 15%. So having four candidates, appealing to different parts of the electorate, with the ability to generate enough support to rack up delegates and remain in the race for a long time, is a recipe for a contested convention.