In a U.S. election year in which personal attacks have crowded out debates about policy, it may seem surprising that the issues might actually determine the outcome of any race. But in New Hampshire, where candidates stand on a controversial energy infrastructure from Canada may decide which party ends up controlling the U.S. Senate and gets the keys to the governor's mansion.

New England-based Eversource Energy's proposed $1.7-billion (U.S) Northern Pass Transmission project would deliver 1,100 megawatts of hydroelectricity from Quebec to customers in New Hampshire and neighbouring states. Its promoters, including Hydro-Québec, bill the project as a stable source of affordable, clean energy for a region that has some of the highest electricity prices in the United States.

While Northern Pass awaits approval from state and federal regulators – and ultimately a presidential permit from whoever enters the Oval Office in January – the transmission line has sparked one of the hottest political debates the state has seen in years.

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The main criticism is that the 310-kilometre Northern Pass line would run through some of the most stunning landscapes in eastern North America. Earlier opposition forced Eversource to agree to bury about a third of the line, a concession that significantly raised the cost of the project.

Some politicians are now calling for the entire line to be buried. Eversource says that would cost up to an additional $1-billion, making the project uneconomical.

Republican gubernatorial candidate Chris Sununu strongly backs Northern Pass in its current form. His Democratic rival, Colin Van Ostern, however, wants the state to rely more on wind and solar power. The race is tight, with two Wednesday polls depicting it as a dead heat.

No race is as closely watched, however, as the U.S. Senate contest involving Republican incumbent Kelly Ayotte, who is running a TV ad in heavy rotation calling on Eversource to bury all of the Northern Pass line and accusing her Democratic opponent of taking "dirty money" from a union backing the project. Ms. Ayotte also charges that her rival, outgoing state Governor Maggie Hassan, "changes her tune" on Northern Pass to suit her audience.

The New Hampshire Union Leader, the state's top daily newspaper and a supporter of Northern Pass, has levelled the same criticism against Ms. Hassan. She "has been entirely consistent" on the transmission project, it wrote in an editorial. "She's in favour of whichever side you're on."

Ms. Hassan has indeed been less categorical than her GOP opponent, saying only that Eversource has "more work to do" with respect to burying Northern Pass's lines. Late in the campaign, that ambiguity has come back to haunt her as she's watched her early lead evaporate.

It's not the first time Ms. Ayotte and Ms. Hassan have clashed over energy infrastructure from Canada. They were also on opposite sides of the Keystone XL oil pipeline -- only, Ms. Ayotte supported it, while Ms. Hassan was steadfast against.

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Polls show the race is a statistical tie. With Democrats needing to gain just four seats to take control of the upper chamber of Congress – five, if Democratic presidential nominee Hillary Clinton loses – both parties are investing heavily in the New Hampshire race.

Ms. Hassan was forced to return $24,000 of a $25,000 donation from the International Brotherhood of Electrical Workers political action committee after it was ruled the initial sum exceeded the limit allowed for a declared candidate. The IBEW counters that Ms. Ayotte's position on Northern Pass is driven by the $44,000 in donations she has taken from Wall Street investment firm Blackstone Group, the lead investor behind the proposed TDI New England Clean Power Link, a competing transmission project that would run from the Canadian border under Lake Champlain, then entirely underground through Vermont.

Hydro-Québec has partnered with Eversource. But TDI's backers would like nothing more than to see Northern Pass rejected, forcing Hydro-Québec to do business with them instead.

Innu from Quebec's Pessamit First Nation have also been lobbying New Hampshire voters to reject Northern Pass, warning that Hydro-Québec's dams on the Betsiamites River have led to fluctuating water levels that damage salmon habitats. Hydro-Québec insists that Northern Pass will have "no impact" on the Betsiamites River and that rules are in place to protect the salmon.

Still, environmentalists opposing Northern Pass argue that Eversource's characterization of Canadian hydro power as "clean energy" is misleading, given the environmental footprint of large dams and the hundreds of kilometres of transmission lines needed to carry the electricity to market. Massachusetts, Connecticut and Rhode Island this week rejected Canadian hydro power in a request for proposals to secure 460 MW of clean energy. Northern Pass is hoping to have better luck next year, when Massachusetts will issue a request for proposals that includes Canadian hydro as an option.

Before then, Northern Pass may have made and unmade more than a few political careers.