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.