"Also we have some of the wealthiest communities in Massachusetts that provide tremendous leadership and are home to many of our great innovators who have been so successful," she added. "So it's a remarkably diverse district, not captive to any one interest, which I have always found to be a real strength of it."

"On the other hand, people are also surprised to learn that there are over 900 farms in this district, so it has a rural quality to it as well," Tsongas said during an interview in her office, in one of the refurbished mills in Lowell.

In many ways, the cities are a lot like Ohio, Wisconsin and Michigan — former centers of manufacturing trying to find new sources of jobs.

The heart of the district is Lowell. It's the biggest city, and it turns out the most voters. Like Lawrence and Haverhill in the Merrimack Valley, and like Gardner, Fitchburg and Clinton in the western part of the district, Lowell still has a lot of traditional blue-collar voters.

Niki Tsongas, the Democrat who has held the seat since 2007, is retiring.

The only race for an open congressional seat in Massachusetts this year is in the 3rd district .

"[I]t's a remarkably diverse district, not captive to any one interest, which I have always found to be a real strength of it."

"You hear a lot of euphemisms apply to this district for non-college-educated white people: hardscrabble, hard-working, tough, flinty," said John Cluverius, a University of Massachusetts Lowell assistant professor of political science. "These are all words that we want to use to describe white people who live in Lowell and Lawrence and maybe Fitchburg and some of the western towns, but that is not this entire district. There are more non-native residents in this district than most congressional districts."

The 3rd district has the highest percentage of Latino voters of any in Massachusetts.

Lowell has the second-largest Cambodian-American community in the country, second only to Long Beach, California. And they are making themselves felt in politics. The Lowell city councilor elected with the most votes is Cambodian-American. And in one race for state representative, three out of the four candidates, including the incumbent, are Cambodian-American.

Immigration is a big issue.

"For example, at the [Lowell] high school, 55 languages are spoken from students coming from around 65 countries," Tsongas said. "So immigration is obviously of deep concern, but then there are [people in] other parts of this district who are more concerned with issues like our inability to come to grips with global warming or the kinds of expenditures we need to make around health care."

Another big issue is the cost of housing. At a forum in Lowell, one of the 10 Democratic primary candidates, hotel entrepreneur Beej Das, described how the explosive growth of UMass Lowell has contributed to the rising cost of finding a place to live in the city.

"Gentrification and the success of the university has really helped this problem," Das said. "That's the reason we're kicking out a lot of people who can't afford their housing anymore. I live a couple of blocks from here, and I'm sad to say I'm part of the problem. My rents rival those I would pay in Boston."

The 3rd has traditionally been a centrist district, one that supported Republicans when the rest of the state went for Democrats. The towns now in the 3rd voted for Republican Charlie Baker over Democrat Deval Patrick in 2010, Republican Scott Brown over Democrat Elizabeth Warren in 2012, Republican Gabriel Gomez over Democrat Ed Markey in 2013.

But two years ago, the district's Democratic primary voters proved to be more liberal than Democrats in much of the rest of the state. Statewide, primary voters chose Hillary Clinton over Bernie Sanders. But in the 3rd district, Sanders won.

The forum in Lowell attracted people newly interested in Democratic politics. People like Shari Bennett, who came from Littleton, three towns away.

"I have never been politically motivated until this presidency, and now I want to learn everything I can," she said. "I'm getting very involved in what's going on now."

The Democratic candidates are leaning left.

"I would say that the 10 candidates running on the Democratic ticket definitely are more progressive," said state Sen. Barbara L'Italien, one of the candidates. "Everyone's talking about single-payer Medicare for all."

At a house party in Groton, liberal silver-haired guests posed a broad array of questions to another Democratic candidate, former Ambassador to Denmark Rufus Gifford: Does he support a draft? How would he support public education? What would he do to bring the working middle class back to the Democratic Party?

He reminded them that not far away is a very different part of the district.

"When you're in the far western part of this district, you can imagine that the politics are very much like Southern New Hampshire," Gifford told the guests. "It gets very libertarian in a lot of ways, and you see people and you hear their stories, and they break your heart, because they are people who have been left behind."

Also running on the Democratic side: Jeff Ballinger, Alexandra Chandler, Leonard Golder, Daniel Koh, Bopha Malone, Juana Matias and Lori Trahan.

Along with independent Mike Mullen, who has qualified for the ballot, whoever wins the Democratic primary will face Republican online auto parts entrepreneur Rick Green, who is from Pepperell, in that more conservative part of the district.

"The questions I get are: What are you going to do about the Rourke Bridge here in Lowell?" Green said outside the minor league ballpark in Lowell, where he was greeting voters. "What are you going to do about Route 2? It's ancient. Of course, the opiate crisis, which hits everybody."

If the last general election is an indication, Green faces a tough race.

Clinton won the district by a margin of 23 points against Donald Trump.