Earlier this month, Marci Young duked it out with four other candidates in a Democratic primary for a Vermont House seat. She came in third in a two-seat district centered around Morristown and Worcester, losing by 215 votes.

But Young had been nominated in July by the local Progressive Party committee. That means she'll be back on the ballot in this November's general election as a Prog, competing against two Democrats who defeated her in the primary, along with a Republican and an independent.

The 53-year-old Morristown resident didn't think twice about taking another shot at the legislative seat.

"I'm determined to speak for unrepresented people," she said. "Nobody [else] is talking about getting corporate contributions out of politics."

Some Democrats see such a strategy as gaming the party system — and risking handing the election to a Republican.

"I think it's a bit disingenuous," said Vermont Democratic Party executive director Conor Casey. "I would say, 'Choose a party and stick with it.'"

Young is one of three House candidates across the state who lost in a Democratic primary but are returning as Progressives in the general election. Casey thinks the law should be changed to bar candidates from double-dipping.

Progressive Party chair Emma Mulvaney-Stanak defends the practice.

"Vermonters want more choice, not less," she said. "There are serious limitations to the two-party system."

It's the latest development in the tense relationship between Vermont Progressives and Democrats that has been evolving for decades.

A growing number of candidates are opting to straddle the two parties, noting that Democrats and Progressives share many policy goals. At times, the parties have worked collaboratively, but feelings of mistrust remain.

The dustup over the House races came as Ds and Ps delicately negotiate how strongly Democrats will support Sen. David Zuckerman (P/D-Chittenden), a longtime Progressive who earlier this month won both parties' nominations for lieutenant governor.

Perhaps it is only natural that, in the year when Sen. Bernie Sanders (I-Vt.) challenged the national Democratic Party status quo with his run for its presidential nomination, a similar political tug-of-war plays out in his home state.

The Vermont Progressive Party traces its roots — and its tension with the Democratic Party — to Sanders, who knocked a Democrat out of power with his surprise 1981 election as mayor of Burlington.

Though Sanders never ran under the Progressive banner, his political protégés did. First called the Progressive Coalition before becoming the Vermont Progressive Party in 2000, the group of left-leaning political activists has grown from its Burlington base to a statewide force. Today, six House members, three senators and state Auditor Doug Hoffer carry the Progressive label.

But some wonder whether Progressives are building a party or borrowing one.

Terry Bouricius, a former Burlington city councilor and state representative, was among the Progressives' first successful candidates. The party was meant to be an alternative to the two major parties, he noted, and he'd like it to stay that way.

"The Democratic and Republican parties are fundamentally controlled by corporate interests. The parties themselves are unreformable," Bouricius said.

A generation after he served, Progressives increasingly rely on a fusion strategy to get elected. No Progressive has ever reached an office higher than the House without running in a Democratic primary.

"I worry that it's not a long-term strategy that can build a strong, independent third party," Bouricius said.

Mulvaney-Stanak argues that Progressives are criticized no matter what strategies they use.

"You can't please people," she said.

The Democratic-Progressive fusion strategy dates back more than a decade. In 2003, Burlington's Progressive mayor, Peter Clavelle, added the D to his ballot line while running for his last citywide term.

"By and large, I shared the same political perspective as those left of center in the Democratic Party," he said.

Progressives considered him a sellout, and Democrats questioned his credentials, Clavelle recalled.

In the next year, 2004, Clavelle ran for governor solely as a Democrat. Though the party shift raised eyebrows, Clavelle faced no opposition in the Democratic primary. Few were interested in the chance to be soundly defeated by first-term Republican governor Jim Douglas

Fusion candidacies go over more easily, Clavelle said, if the candidate plays nice with the borrowed party. "If you opt to work with the party, I think there has to be a truce of sorts," he said. "You need to be diplomatic."

Truces and diplomacy can be tricky, however. Years of hard feelings linger as Ps and Ds each blame the other for past transgressions.

In 2002, Progressive Anthony Pollina and Democrat Peter Shumlin split the liberal vote in a lieutenant gubernatorial race that Republican Brian Dubie won.

When Pollina ran for governor six years later as an independent, he reveled in outperforming Democrat Gaye Symington, even though Republican Douglas soundly defeated both.

In 2010, Progressives opted against running a candidate for governor to clear the way for Shumlin, only to watch the Democrat later abandon his promise to provide universal health insurance. Progressives still feel so burned by Shumlin that the party this year declined to back Democratic gubernatorial candidate Sue Minter — or anyone else in the race.

Into this arena steps Zuckerman, looking for biparty unity as he runs for lieutenant governor. A longtime Progressive who once railed against Democrats, Zuckerman used the hybrid-party approach twice to win a Chittenden County Senate seat. He argues that it's a sensible strategy that acknowledges that the two parties share many policy goals but also maintain distinct identities.

"We work within the system to make as many changes as we can," Zuckerman said. "We also push the envelope outside the system, because the system doesn't always allow for change."

The Progressive party deftly handled his primary process. They sent an email to supporters asking for 250 volunteers to vote Progressive to ensure that Zuckerman would win that nomination. Meanwhile, they encouraged other Progs to vote for him in the Democratic primary.

Though Zuckerman handily won the Democratic nomination for lieutenant governor, he's still navigating the two parties' complicated relationship. The day after the primary, Democrats locked arms with Zuckerman at a Burlington unity rally while simultaneously declaring that they were unsure about giving him access to their voter database.

Just Monday, however, Zuckerman and the Vermont Democratic Party reached an agreement. The party will not share its full voter database, but will give him access to field offices, share information about voters collected during this campaign and aid Zuckerman with its get-out-the vote efforts.

"This is 100 percent uncharted territory," said Meg Polyte, Zuckerman's campaign manager and a longtime Progressive activist. "Progressives have never run a candidate who collaborated this much with the Democrats... What we all came down to was, there's a lot of mutual value."

"The wide feeling is, we can't afford to be divided," Casey said.

But he quickly cautioned that the pact is between the Democratic Party and Zuckerman's campaign — not the Progressive Party. "We'd be very wary of having data collected by Democrats over the years falling into the hands of a party that's distinct from ours. That's not going to be the case here," Casey said.

Meanwhile, some established Democrats are embracing party fusion. Two incumbent senators — Phil Baruth of Chittenden County and Dick McCormack of Windsor County — this year sought and won the Progressive nomination. They'll run for the first time as Democrat/Progressive.

Baruth acknowledges that because he's the Senate's Democratic majority leader, the move is significant.

"I'm saying as publicly as I can that I'm proud to work with Progressives," he said. "It's a healthy thing for the two parties to work together."

Fusion first came to the Senate in the 2008 election, when Sen. Tim Ashe (D/P-Chittenden), then a Progressive Burlington city councilor, employed the strategy to win a seat. Pollina followed suit two years later — and Zuckerman two years after that.

Ashe said it's been effective in reducing head-on challenges between Democrats and Progressives.

"When I first ran for Senate, Democrats and Progressives were running against each other in statewide races," Ashe said. "We almost never see that anymore."

The proliferation of fusion candidates mirrors a leftward shift in the Senate. It's conceivable that next year, three P/Ds could be running the institution — if Zuckerman is elected lieutenant governor, Ashe is chosen as Senate president pro tempore and Baruth remains majority leader.

Fusion doesn't seem to be all-inclusive. Three Democratic Chittenden County senators — Michael Sirotkin, Ginny Lyons and Ashe — won enough write-in votes in this month's primary to earn the Republican nomination, but they chose to decline it.

In House races, Progressives have ramped up the fusion strategy. This year, 21 Progressive House candidates ran in Democratic primaries. Fifteen of them won and will carry both the Democratic and Progressive label in the general election.

But it's those who lost in Democratic primaries and are running in the general election as Progs who are causing a stir.

Carl Etnier came in second in a five-way Democratic primary for an open House seat in East Montpelier — 219 votes behind winner Kimberly Jessup. Etnier plans to return as a Progressive in November.

"I think I have a good chance of winning," he said. "A lot of people voting in November didn't vote in the primary."

Etnier, a former Progressive Party State Committee member and town chair, said he's been active in both parties, including as a phone bank volunteer for Minter, the Democratic gubernatorial candidate.

But straddling parties isn't easy. Both demand loyalty. Etnier won the Progressives' support by pledging allegiance foremost to that party.

Traven Leyshon, chair of the local Progressive committee, said the group chose Etnier over Jessup because he promised to caucus with the Progressives, while Jessup said she would caucus with Democrats. Democrats are too willing to compromise on issues such as universal health care, Leyshon argued.

Jessup, a first-time candidate seeking to replace retiring Rep. Tony Klein (D-East Montpelier), said she wanted to work with both parties, but she didn't waver on which came first.

"I ran in the Democratic primary because I am a Democrat," she said.

Leyshon said the Progressive committee weighed whether Etnier's presence on the ballot could split the ticket and help a Republican win. The panel decided the Republican wasn't a credible threat, he said.

Nonetheless, Klein was infuriated that Etnier had chosen to "take two bites at the apple."

"If you're a Progressive because you seem to be dissatisfied with what the other parties represent, then you should run in the Progressive primary," Klein said.

A similar sentiment emerged with Young's candidacy in the Morristown/Worcester district.

There, Republican Gary Nolan could prevail if the two Democrats and the Progressive split the liberal vote, according to Rep. Avram Patt (D-Worcester), who, with David Yacovone, won the Democratic nomination in the two-seat district.

Young defended her Progressive candidacy, saying she didn't get a fair shake in the Democratic primary because she was excluded from a candidate forum the party hosted.

Ginny Burgess, the Morristown Democratic Town Committee chair, said Young was kept out for good reason. Organizers asked the candidates to commit to supporting whoever won the Democratic primary, and Young declined.

"This was a forum for Democratic candidates," Burgess said. "It's simple."

With Democrats and Progressives, it's never simple.

Disclosure: Tim Ashe is the domestic partner of  Seven Days publisher and coeditor Paula Routly.

Correction, August 25, 2016: Marci Young was nominated by the local Progressive Party committee for a House seat on July 7, and the Vermont Progressive Party was established in 2000. An earlier version of this story contained incorrect information.