The Democratic primary for governor in Maine will head to a runoff under the state’s ranked-choice voting system, The Associated Press said on Tuesday.

The candidates advancing to the runoff are Shenna Bellows, Maine’s secretary of state; Troy Jackson, a logger from northern Maine and former state senator; Hannah Pingree, a former state representative who went on to work for the administration of Gov. Janet Mills; Dr. Nirav Shah, who led the state’s response to the coronavirus pandemic.

Maine uses ranked-choice voting for primary races with multiple candidates, allowing voters to list their choices in order of preference. Because no Democrat was projected to receive more than 50 percent of the vote, according to The A.P., the race now moves to what is effectively an instant runoff.

The ranked-choice voting system uses successive rounds of counting, eliminating last-place candidates and awarding their votes to a voter’s next choice instead, until one candidate receives more than 50 percent. Election officials will confirm the winner in the coming weeks.

A fifth candidate, Angus King III, a businessman and son of Senator Angus King, was projected to place last in the race. According to the Maine secretary of state’s office, the candidate with the fewest first-place votes will be eliminated from the runoff.

For months, the crowded, collegial race had at times barely qualified as background noise beneath the din of a high-stakes Senate campaign in which Graham Platner, a populist Democrat, is challenging Senator Susan Collins. All five Democrats described similar policy goals, including expanding access to health care and housing. In debates, they have highlighted their common ground more often than they have clashed; most have also sought to prove they will be champions of the working class.

Mr. Jackson frequently talks about lacking health insurance in the past, and reminds voters of his roots in rural Aroostook County, a conservative northern region. Ms. Bellows has emphasized her own upbringing in rural Hancock County, where her father was a carpenter and where she lived without electricity or running water until fifth grade.Several polls this year had shown Dr. Shah in the lead, but the race tightened last month after Mr. Platner said he had ranked three of Dr. Shah’s rivals as his top choices in the governor’s race: Mr. Jackson, Ms. Bellows and Ms. Pingree.

After winning Mr. Platner’s seal of approval, his three preferred candidates announced an alliance, urging voters who chose one of them to also rank the other two. Mr. Jackson, in particular, aligned himself with Mr. Platner, sticking with him even as revelations emerged recently about some of Mr. Platner’s past relationships and interactions with women.

Mr. Platner won the party’s nomination on Tuesday.

Dr. Shah, who spent most of his career in the Midwest, came to Maine in 2019, when Gov. Janet Mills hired him as the state’s public health director. In an interview days before the election, he said that he had heard from many voters seeking leadership from outside the “machinery” of Maine’s political establishment.

Ms. Mills, a Democrat who is term-limited, endorsed Ms. Pingree, who is the daughter of U.S. Representative Chellie Pingree.