Hannah Pingree, a former Maine House speaker who later worked on policy initiatives for Gov. Janet Mills and won her endorsement, won the state’s Democratic nomination for governor after a ranked-choice runoff, state officials said early Friday. She will face Bobby Charles, a conservative Republican and former State Department official.

The runoff, a feature of Maine’s ranked-choice voting system, was necessary because none of the candidates in either primary received more than 50 percent of the vote. Mr. Charles had led his race after the primary last week, with 38 percent of the vote, making him the heavy favorite to win the nomination. But among Democrats, the top four candidates were closely bunched.

Dr. Nirav Shah, who led the state’s response to the Covid pandemic, was in first place at the start of the runoff, with 27 percent of the vote. Ms. Pingree had been in second place, with 23 percent. Troy Jackson, a logger from northern Maine and former State Senate president, and Shenna Bellows, the secretary of state, each had 21 percent.

In ranked-choice voting, where voters can rank multiple candidates in order of preference, the runoff process is unpredictable, giving all but the last-place finisher in the election a shot at winning. The runoff took days because law enforcement had to transport ballots from across the state to Augusta, the capital. Then elections officials uploaded them into a computer system, and checked vote tallies from municipalities to ensure none had been lost.

Finally, in the early hours of Friday, a state official pushed a button, and a computer rapidly performed successive rounds of tabulations in which last-place candidates were eliminated and their votes were awarded to each voter’s next choice. That continued until one candidate, Ms. Pingree, received more than 50 percent.

Ms. Pingree has campaigned as a progressive, with a focus on making housing and health care more affordable. She was one of three Democratic candidates for governor who forged an alliance with Graham Platner, the populist Democrat running for U.S. Senate in Maine.

Several polls this year had shown Ms. Pingree in the middle of a crowded Democratic field, but the race tightened as the primary drew near. Her name is a familiar one in Maine, where her mother, Chellie Pingree, has represented the southern part of the state in Congress since 2009.

Ms. Pingree’s strong showing in the primary appeared to signal enthusiasm from Maine Democrats for more progressive leadership in the governor’s office. Ms. Mills, a Democrat who is term-limited, has taken a politically moderate approach to governing, sometimes frustrating progressives.

Despite the close ties between the nominee and the governor, Ms. Pingree has said she disagrees with some of Ms. Mills’s positions. She has said that she would sign legislation to help Maine tribes achieve sovereignty and gain access to federal benefits, for example; Ms. Mills vetoed such legislation in 2023, citing concerns about lawsuits.

In the crowded Republican primary field, made up mostly of business leaders, Mr. Charles, a lawyer, was consistently the front-runner. His closest competitors were Jonathan Bush, a cousin of former President George W. Bush, and Benjamin Midgley, a former president of Planet Fitness.

Mr. Charles, a blunt-spoken conservative in the mold of President Trump, railed against fraud and corruption in state government during his primary campaign, and spoke frequently of his Christian faith. He pledged to stop illegal drugs from coming into Maine and to aggressively cut taxes and state spending. Last month, he released a plan to “prevent the Islamification of Maine,” where a number of asylum seekers from African countries have settled.

Ms. Pingree was raised on the island of North Haven, a dozen miles off the coast of mainland Maine. The product of a wealthy family with deep roots in New England, she quickly gravitated to politics. She won election to the Maine House of Representatives at 26, served eight years and became the youngest woman in the country to be chosen as House speaker.

As director of policy innovation in the Mills administration, Ms. Pingree helped draft the state’s climate action plan, released in 2024, which aims to defend critical coastal infrastructure from increasingly extreme storms, reduce greenhouse gas emissions and mitigate rising heating costs in the oil-dependent state.

The governor’s race has been largely overshadowed by Mr. Platner’s high-profile campaign. Mr. Platner, now the Democratic nominee, is trying to defeat Senator Susan Collins, a Republican seeking her sixth term. In the governor’s race, large fields of candidates and heavy overlap in their positions led many voters to postpone their decisions about how to rank them.