Sen. Bernie Sanders accused Nissan's North American branch of union-busting as workers at a Mississippi plant began a two-day vote on joining the United Auto Workers.

The pre-emptive nature of the attack suggests that labor allies fear that the UAW's bid will go down in defeat when the votes are counted.

"This could go down as one of the most vicious, and illegal, anti-union crusades in decades. Workers should never have to endure this type of threatening campaign or walk through a minefield just to vote for a union," Sanders said in an op-ed for the London-based Guardian.

About 3,500 to 3,800 Nissan workers at the company's Canton plant will be eligible to vote in the election, which started Thursday and will end Friday. Another 3,000 plant workers will not be eligible as they do not work directly for Nissan.

Unions have long struggled to build support in the labor-averse South, a task that has become more urgent in recent years as foreign manufacturers, particularly auto companies, have increasingly located there. UAW's membership stands at 415,000, according to its most recent Labor Department filings. That's down from 464,000 a decade ago and more than 700,000 in 2001.

UAW was able to secure enough worker signatures to force a federally monitored organizing vote – 30 percent of eligible workers is required – but the union has not claimed to have majority support. Nissan has suggested the election may be a pretext and the union doesn't expect to win this time. The union's real goal, the company has suggested, is to gain the workers' contact information, which the company must turn over when a vote is ordered regardless of whether individual workers consent.

In the lead-up to the vote, the UAW has asserted that Nissan has engaged in an extensive union-busting campaign. Last week, the National Labor Relations Board, the main federal labor law enforcement agency, filed an unfair practices complaint against the company based on complaints lodged by the UAW. Sanders' column echoes those complaints, claiming that the automaker is "deluging employees with anti-union literature and is threatening to close the plant."

Sanders' op-ed does not indicate that he thinks the workers' drive for a union is strong enough to overcome that. "Regardless of what happens this week, Nissan workers should be very proud. They have exposed the system of racial and economic injustice that corporations like Nissan are perpetrating."

Nissan has said it has merely tried to keep workers informed: "The UAW has advocated employees only hear one side of the story – the union's side – and that's wrong. The company has the right, and we believe the obligation, to provide employees with information as they prepare to make this important decision, and we will continue to do so," it said in a statement.

Most southern states, Mississippi included, have right-to-work laws, which prohibit workers from being forced to join or otherwise financially support a union as a condition of employment, a common provision of labor-management contracts in northern states. The laws are widely seen as a major reason why organized labor has weak support in the South.