In rural western North Dakota, tens of thousands of men work in the lucrative oil fields, far away from their families sprinkled across the country.

The residents of these “man camps” may not know it, but they could decide which party controls the Senate.

Only one or two races will determine the Senate majority in November’s elections — and the North Dakota race is one of the most competitive. But it’s the state’s lax voting requirements and small population that make nomadic oil-field workers politically relevant.

That’s why the candidates — former state Attorney General Heidi Heitkamp (D) and Rep. Rick Berg (R) — started working camps that house transient workers months ago.

“Not a week goes by where you don’t hear about Heidi Heitkamp or Rick Berg stumping in the oil fields, visiting Walmarts, a coffee shop or ‘man camps’ where these oil workers live,” said Patrick Davis, a Republican consultant who works in the state.

In larger states, it’s an unproductive chore to target and register unreliable voters such as these. But North Dakota has the least cumbersome voting requirements in the country.

It’s the only state without voter registration. Residents must only show proof of residency for the last 30 days, such as a utility bill or driver’s licence, to cast a ballot.

If voters don’t have proof of residency handy, they can sign an affidavit confirming residency — or one of the local poll workers can vouch for residency in a special form. What’s more, half of the state’s counties offer vote by mail with the same affidavit.

For months, polls have shown the race between Berg and Heitkamp as a statistical tie. That means every vote counts in the pool of about 350,000 voters expected to cast ballots in the open-seat contest.

It’s difficult to determine the exact number of transient oil workers. But North Dakota’s population has increased by 44,000 since 2008, according to the U.S. Census Bureau. Experts say most of that growth is in the rural western part of the state.

The explosive growth has led to a housing shortage. Workers with license plates from as far away as Colorado, Texas and Louisiana reside in small trailer camps. The workers often commute for two weeks at a time to North Dakota before returning home.

It’s almost impossible to poll that population. But operatives believe the oil workers lean Republican, given their profession, and so the GOP is seeking to capitalize on the temporary residents.

Republicans started a super PAC, Brighter Future Fund, to target this specific population. Shane Goettle, who runs the super PAC, said it has a budget of $60,000 to educate oil workers on voting requirements through radio and literature.

“We think that it’s probably to [Berg’s] advantage to have oil fields voters voting,” said Goettle, a Berg supporter. “The size of that advantage is hard to measure.”

But Heitkamp — a former energy company director at Dakota Gasification Co. — is also fighting for votes among the rural population. Her main campaign office is in Mandan, a city near Bismarck in the western part of the state. Berg’s team is based in Fargo, which is on the eastern border. She has also pledged to open a Congressional office near the oil fields if she’s elected.

More urgently, both campaigns are preparing for what could be a minefield of legal challenges based on the easy voting process — especially if the race is close on Election Day.

The Democratic nominee’s campaign has recruited attorneys for western North Dakota’s two main population centers: Williston and Dickinson.

“We have a legal team of North Dakota attorneys ready and in place and already have begun recruiting attorneys to monitor key districts to ensure that every North Dakotan who can vote is able to vote, and that includes monitoring Dickinson and Williston,” Heitkamp spokesman Brandon Lorenz said.

Republicans made similar legal moves, contacting attorneys in early summer to be poll watchers, checkers and challengers, according to local party officials.

Local election workers can flag suspect ballots and send them to the state’s attorney to investigate, according to Lee Ann Oliver, an elections specialist in the North Dakota secretary of state’s office. She said the state’s attorney can also investigate ballots at will.

And for the first time, Oliver said, North Dakota will notify other states about who votes. That way, she said, voters can vote in only one state. But it’s a process the state has just started to implement and could take time to finish after Election Day. If the race is close, as most operatives expect it to be, a litigious battle could ensue. Operatives compared the possibility to the 2008 Senate race recount in Minnesota — on steroids.

After all, North Dakota hasn’t had many competitive Senate races recently. The last time a race was this close? When retiring Sen. Kent Conrad (D) won his first term in this seat in 1986 by less than 1 point.