Voters in north-central Massachusetts will cast ballots Tuesday in one of the cycle’s most crowded primaries. Democratic Rep. Niki Tsongas is retiring after five full terms, creating a free-for-all for the 3rd District, the state’s only open seat this year.

The ten Democrats running include the first openly transgender person to run for Congress in Massachusetts, two candidates who immigrated to the United States as children, and a former ambassador whose experience as a gay diplomat was chronicled in a Danish television documentary. Winning the primary would be tantamount to victory in the heavily Democratic seat that Hillary Clinton carried by 23 points in 2016. 

The sheer number of hopefuls in the Merrimack Valley district has made it difficult for any to stand out. That’s partly because they share many policy views. And almost all tout eye-catching résumés studded with the names of elite political figures and institutions. 

Leading the pack in fundraising has been Daniel Koh, a former chief of staff to Boston Mayor Martin Walsh, who raised over $3 million through Aug. 15, the end of the pre-primary reporting period and had over $800,000 in the bank. Of Korean and Lebanese descent, Koh holds bachelor’s and MBA degrees from Harvard and could be the first Asian-American congressman from Massachusetts. He led the field in a recent UMass Lowell-Boston Globe poll, drawing 19 percent of support from likely voters. 

But other contenders, such as state Sen. Barbara L’Italien, have seen an opening in a lawsuit filed by a former city employee who complained of the way Koh responded to her sexual harassment allegation against another employee. 

“That was his chance to stand up and do the right thing, and he didn’t do it,” L’Italien said this week on Boston Herald Radio. 

Koh’s campaign accused L’Italien of twisting the facts, saying the mayor’s office “took these allegations very seriously and responded quickly.”

L’Italien was tied for second place at 13 percent in the UMass Lowell-Globe poll along with Rufus Gifford, the former U.S. ambassador to Denmark during the Obama administration. She generated some national buzz in July when she was mistakenly booked on a Fox News interview and used the opportunity to criticize President Donald Trump. She’s raised almost $800,000 through Aug. 15 and had $186,000 on hand. 

Gifford was hit by an anonymous video attack last weekend that his campaign said selectively edited comments he made in the Danish TV documentary to portray him as an out-of-touch outsider. A longtime Democratic fundraiser, he’s been endorsed by the LGBT Equality PAC and the LGBTQ Victory Fund. He raised $2.2 million through Aug. 15, and banked almost $520,000.

Business consultant Lori Trahan, a onetime chief of staff to Tsongas’ predecessor, Democrat Martin T. Meehan, has endorsements from The Boston Globe and the Boston Herald. She’s raised $1.2 million through Aug. 15 and had $480,000 on hand. 

Trahan was in fourth place in the UMass Lowell-Globe poll with 8 percent. State Rep. Juana Matias, an immigrant from the Dominican Republic who’s backed by BOLD PAC and the Congressional Black Caucus PAC, took 6 percent. Alexandra Chandler, a transgender woman and former intelligence analyst at the Pentagon, was next with 4 percent. 

Hotel executive Beej Das, labor activist Jeffrey Ballinger, lawyer Leonard Golder and former bank executive Bopha Malone, who came to the U.S. as a refugee from Cambodia, round up the Democratic field. 

Vacancies in the state’s all-Democratic House delegation have been rare, which could explain the high number of candidates running to replace Tsongas. 

“What I think happened is that ten people really squinted hard at this election and said, ‘I can take a run at this,’ because they didn’t see an obvious front-runner emerging and there wasn’t a sort of consensus among elites in the party or donors about who that candidate should be,” John Cluverius, associate director of the UMass Lowell Center for Public Opinion, told The Associated Press.  

The race is also complicated by what many have predicted will be sluggish turnout after Labor Day weekend.

And 29 percent of likely voters were still undecided, according to the UMass Lowell-Globe poll, an indication that many of the candidates were still struggling with name identification.  

“Ten candidates in a congressional district primary leads to information overload, which can lead a voter to shut down and stop paying attention,” said Joshua Dyck, co-director of the UMass Lowell Center for Public Opinion. 

Businessman Rick Green, who is running unopposed in the Republican primary, will be the heavy underdog against whichever Democrat emerges from Tuesday’s primary. After defeating her GOP opponent by 6 points in a 2007 special election that brought her to Congress, Tsongas won her subsequent re-elections by double digits.