PORTSMOUTH, N.H. — In what’s been dubbed the "swingiest" House district in the country, voters will cast their ballots Tuesday to pick which candidates will face off in November’s general election.

New Hampshire’s 1st Congressional District has flipped back and forth between the same Democrat and Republican representatives every single election for the last decade, but with Democratic Rep. Carol Shea-Porter’s decision to retire, this year's race is wide open.

"Donald Trump and John Kelly should be watching what happens in the 1st Congressional District, because this is definitely going to be a bellwether to what happens in the future," said Neil Levesque, the Executive Director of the New Hampshire Institute of Politics at Saint Anselm College.

New Hampshire voters are used to getting attention as the first state in the nation to cast primary ballots in presidential election years. But the stakes in this year’s midterm election couldn’t be more pronounced. In 2016, it was one of the few districts in the country carried by both Donald Trump and a Democratic congressional candidate — both of them winning the area by just one point.

The Democrats

The district’s peculiar position goes beyond its swing status this year with a number of notable dynamics at play among the candidates themselves.

There is a large field of candidates on the Democratic side, including Shea-Porter’s former Chief of Staff Naomi Andrews and Bernie Sanders’ son Levi Sanders. But analysts in the Granite State view the top two Democratic contenders as Executive Councilor Chris Pappas and Marine veteran and former Veterans Affairs official Maura Sullivan.

At first glance, Pappas appeared to be an early favorite since he’s been a face in the area’s Democratic politics for years. He boasts support from much of the state’s Democratic establishment, including both Senators Jeanne Shaheen and Maggie Hassan. He’s also familiar to patrons of the Puritan Backroom — a business in his family for the last century, one of the region’s more popular restaurants, and a go-to stop for potential presidential candidates.

But Sullivan, who just moved to New Hampshire last year, has out-raised all of the competition with a significant sum of cash that has primarily come from out of state, a distinction Pappas has tried to emphasize. Pappas isn’t shy about focusing on his Granite State roots, saying, "you have to work to understand local concerns, make a connection with people that goes beyond politics. And for me as a local business owner, I think I've got a much easier shot of doing that than other folks."

Sullivan says her recent move to the district doesn’t ultimately matter to voters, and she tends to defend the move as another example of challenging the status quo. "There were a number of people that didn't want me to run, that told me that it was not my turn, that this was somebody else's seat," she said in an interview. "I think women this year are really tired, if you look across the country, of being told that it's not their turn."

Sanders, meanwhile, hasn’t raised nearly as much money as Pappas or Sullivan, and he lives in Claremont, a town that sits on the other side of the state from the 1st District (candidates only need to reside in the state, not the district, to run).

Levi Sanders prior to a debate for Democratic hopefuls in New Hampshire's 1st Congressional District at the Institute of Politics at St. Anselm College in Manchester on Sept. 5, 2018. Charles Krupa / AP

He has spent much of his campaign challenging his fellow Democrats to adopt a "Medicare for All" platform and other progressive stances, but he hasn’t received an official endorsement from his father, who won the New Hampshire presidential primary back in 2016.

"The American public is not interested in dynastic politics," Sanders said in an interview. "People need to earn their place and that’s what I think I’m doing."

The Republicans

On the Republican side, the two candidates who have gotten the most attention are State. Sen. Andy Sanborn and Navy veteran and former police chief Eddie Edwards, who both claim Trump advisers as endorsers and both ran campaigns aligning themselves closely with the president.

But the race between them has them turned dark and bitter, with Edwards refusing to endorse Sanborn if the state senator wins and repeatedly bringing up “character issues” through the course of his campaign.

"I can tell you, here in the Granite State, we care about integrity," Edwards said in an interview. "We care about character."

While it's a district used to frequent changes of party control, Sanborn says this year's crop of candidates makes it a different race than in past cycle. "Part of this whole bellwether thing is when you have the same two candidates that run against each other every two years," he said. "Now we have brand-new candidates on both sides."

The decision

The state could be poised to make history with multiple candidates — Edwards as their first African-American member of Congress, and Pappas as their first openly LGBT congressman.

The ultimate direction the district takes is likely to be chosen by their famous independents — called "undeclared" voters in New Hampshire — estimated to make up more than 40 percent of the state’s voters.

"What you have is the bases turn out, and then you have this group in the middle," said Levesque. "They're choosing who they send to Washington."

That’s made undeclared voters like Rick Charbonneau a top target.“It doesn't matter to me that they're Democrats or Republicans,” he said. "It's the person and what they're going to accomplish and try to accomplish."