Multicandidate races suggest robust engagement in participatory democracy and offer voters a wide range of choices. But the surge of candidates means that in some districts, the winners are likely to win primaries with far short of a majority, raising questions of whether crowded fields bolster democracy or undermine it. In a suburban Philadelphia district, where 10 Democrats competed in a May primary, the winner received less than 29 percent of the vote.

“In these races with lots of candidates, you usually end up with one who wins only a plurality,” said Joshua Dyck, a political scientist at the University of Massachusetts Lowell.

“The danger is that that isn’t necessarily the person who has the greatest support,” he said. “And you’re violating simple principles of majority rule.” Such packed fields are spurring more talk of ranked-choice voting, in which voters rank their choices and a winner must obtain a majority.

Massachusetts has seen crowded congressional primaries before, in which candidates won with little more than 20 percent of the vote. That is the likely outcome in the Democratic primary in the Third District, which stretches from the old mill towns of Lawrence and Lowell to the graceful homes of historic Concord.

The decision by the district’s representative, Niki Tsongas, a Democrat, not to seek re-election has created a rare open seat. Members of the state’s all-Democratic congressional delegation usually stay as long as they want, and once in office, they are almost never challenged. This job security — combined with anti-Trump fervor — helps explain why so many candidates (15 at one point; 11 at last count) have flooded the field.

Except for a brief period in the mid-1990s, the Third has been reliably Democratic since the 1930s, though its boundaries have shifted. The winner of the Democratic primary, on Sept. 4, is all but certain to win in November. Only one person is running on the Republican side.