But those races were the exceptions that proved the rule, so to speak, in a state where cycle after cycle, most incumbents go unchallenged. Of the 200 House and Senate seats on the ballot, only 18 incumbents running for reelection faced a challenge, meaning only a small percentage of the electorate had any choice on Tuesday.

The lack of choice at the ballot box is what one political scientist described as the “most concerning” factor in Massachusetts politics.

“Many people look at the possibility of a successful primary challenge and quit before they’ve even started,” said John Cluverius, an associate professor of political science at the University of Massachusetts Lowell. “The incumbency advantage is real across the country, but the ability to deny voters even the chance to vote against them is much, much higher in Massachusetts.”

Incumbents have long enjoyed staying power on Beacon Hill. Between 2002 and 2022, 79 percent of Democratic House primary races were uncontested, a Globe analysis of state election data shows. In the same period, just 25 Democratic House candidates defeated an incumbent in their primaries.

This year’s political calendar, with primary day immediately after the Labor Day holiday, made it hard for voters to cast ballots. Some are still out of town, and students at the state’s nearly 100 colleges and universities don’t have time to register at new addresses by the Aug. 24 deadline.

There are also countless others who move on Sept. 1, when the majority of apartment leases in the Boston area turn over. In Massachusetts, the deadline to update voter registration information, such as an address, or change political party is 10 days before any election, or before many of those people had moved this year.

Massachusetts law designates the primary to be held on the seventh Tuesday before the general election. That should have meant Sept. 17 this year. But in 2023 lawmakers voted to move up the date at the request of Secretary of State William F. Galvin, a common practice over the last decade-plus. In 2022 and 2018, the primary was held directly after the holiday.

Meanwhile, an effort in 2022 to put Massachusetts on the map with 22 other states with same-day voter registration stalled in the Legislature.

Taken together with the lack of term limits and low rates of retirement on Beacon Hill, “we have a system that is designed to protect incumbents and discourage challengers,” Cluverius said.

“Running an insurgent campaign in Massachusetts is demonstrably more difficult than it is in most other states,” he said.

Amanda Litman, executive director of national group Run for Something, works to promote political newcomers, many of whom are challenging incumbents and are subject to a myriad of other barriers, too.

In primaries, for example, a newcomer may be shut out from accessing voter files, shared tools, or even office space available to other members of their party. Incumbents enjoy existing relationships with endorsing organizations such as labor unions, she said. Fellow elected officials may also hesitate to publicly back a challenger to an existing colleague in a primary.

Litman said despite this dynamic, it’s healthy for incumbents to be challenged, especially in districts where voters may rarely have a choice.

A contested primary “shows that no one is safe,” said Litman, whose group endorsed MacKay, the democratic socialist who faced off against Decker Tuesday.

Decker, in an interview before the election, argued that uncontested primaries signal constituents are happy with their officials.

“I continue to get elected because people trust and appreciate the work I do,” she said. “I deliver and I make laws and I add to the budget. That matters to them. I am deeply grateful that they continue to choose me to this work.”

In the two other primary upsets, the challengers only won after losing a previous effort in 2022.

Mara Dolan, who beat incumbent Governor’s Councilor Marilyn M. Petitto Devaney, ran for the seat last cycle, coming up short by 1,658 votes. So did Tara Hong, who unseated five-term state Representative Rady Mom in Lowell on Tuesday. Last time, Hong lost to Mom by just 56 votes. This year, Hong’s margin of victory was more than 180.

Dolan said she always planned on running for the seat again. This time, however, she spent 18 months campaigning, three times as long as her first run for the Governor’s Council.

“It is difficult to run against an incumbent,” the Concord Democrat said. “If you want to win an election, you need to know it is going to take a tremendous amount of work. . . . Sometimes the stars align. Other times, they don’t.”

Devaney said incumbents such as herself usually get reelected because of their “experience, commitment, and knowing the district you are representing. . . . When you replace someone, you get someone who is not going to have that information or who to go to get things done.”

Mom did not respond to a request for comment.

Hong said his message of serving as a “full-time and active” state representative resonated with voters disenchanted with Mom, who won his first race in 2014.

“There are some parts of the district where they don’t know that we even have a state rep,” Hong said.

Despite the uncertain outcome in the Cambridge race, those on the left celebrated MacKay’s showing against Decker, a high-ranking Democrat endorsed by several members of Congress, the state’s largest unions, and even Governor Maura Healey.

The campaign “was not something that was just focused on getting a specific person elected, but about increasing the attention to the State House in the district,” said Jonathan Cohn, policy director of Progressive Massachusetts, which endorsed MacKay. “That doesn’t go away.”

MacKay had led Decker by 40 votes in an unofficial count late Tuesday before Cambridge election officials on Wednesday hand-counted hundreds of additional ballots, including those from overseas or drop boxes. The new totals show Decker with 3,472 votes to MacKay’s 3,431, a difference of just 41 ballots.

Litman said regardless of the outcome, the challenge will inspire others to run.

“For every Evan who is brave enough to put their name on the ballot, there are countless more who become braver, too,” she said.

A rare upset happened on the Republican ballot, too. In Plymouth County, Republican John Gaskey ousted 11-term state Representative Susan Williams Gifford by leaning further to the right, centering his campaign around his background as a Coast Guard veteran and support for the antiabortion movement.

Because no Democrats ran for the seat, Gaskey will presumptively assume office in January.

“As Republicans, we know the reality of living in a state with a Democratic supermajority. But I got in this race because I believe that Republicans still have a choice — to stand up to the madness and say NO,” Gaskey wrote on Facebook Wednesday.

Cluverius, the professor, said there is one solution “for the monolithic power of incumbency” in Massachusetts: for challengers to run for office and, if they lose, to run again.

“Voters are more likely to reward persistence,” he said.

Samantha J. Gross can be reached at samantha.gross@globe.com. Follow her @samanthajgross.