JACKSON, MISSISSIPPI: The United Auto Workers union charges that Nissan Motor Co. broke federal labour law less than two days before a Mississippi rally where US Sen. Bernie Sanders plans to speak in favor of unionisation.

The UAW said a company security guard wrongfully stopped workers from handing out literature and asking fellow employees to sign cards authorising a union vote outside a gate of the Canton plant Thursday afternoon.

A union lawyer emailed an unfair labour practice charge to the National Labour Relations Board late Thursday. It's one of several now pending over the union's yearslong campaign to represent some or all of the 6,400 people who work at the complex.

Sanders, a Vermont independent, told The Associated Press in a phone interview Friday that pro-union workers are fighting an uphill battle against an unfairly hostile company.

"I think the workers in the Nissan plant in Mississippi have shown an enormous amount of courage," the former candidate for the 2016 Democratic presidential nomination said. "When people stand up and fight for their rights, it's important to stand with them."

Federal law says workers can talk about a union on a company's property, as long as they do it before or after work or during breaks, and in non-work areas such as parking lots or break rooms.

Nissan said the company is investigating but isn't sure yet what happened. Spokeswoman Parul Bajaj notes that unions often file unfair labour practice charges during organising campaigns.

"Nissan abides by US labour law and respects the employees' rights to choose if and how they wish to be represented," Bajaj wrote in an email.

Nissan spokesman Brian Brockman denied that the company is improperly pressuring workers, although he acknowledges that the company talks to workers about the UAW. A plantwide video recorded last year by a union supporter shows a top manager saying the company doesn't believe UAW representation is in Nissan's best interests.

"If he's saying we're intimidating workers or not telling them they don't have a vote, that's simply not the case," Brockman said.

Sanders, though, said it can be "extremely difficult" for workers to form unions.

"In the United States of America, people should be free to vote for a union or against a union without harassment, without threats, without fear of what might happen to them," he said.

Sanders said he was attracted to the Mississippi effort in part because he believes unions could bargain for better pay and benefits, neutralising threats by companies in higher-wage states to move south.

"If we can raise wages in Mississippi and Alabama and some of the other low wage states in the South, we will benefit workers all across the country," he said.

The UAW has long struggled to unionise foreign-owned auto plants across the South. It has lost two votes at Nissan's plant in Smyrna, Tennessee, and no vote has ever been held in Canton.

NAACP President Cornell William Brooks and actor Danny Glover also are scheduled to speak at the rally organised by the pro-union Mississippi Alliance for Fairness at Nissan. The alliance seeks to link support for the union with civil rights for African-Americans.

After the rally, Sanders and others are scheduled to march to the plant to present a letter to the company. An advance copy provided to the AP shows they will demand that Nissan "cease intimidation and threats" against pro-union employees, discuss a neutrality agreement with the UAW ensuring employees can vote on a union "in a free and fair election" and improve training and safety.

Mississippi's political and business establishment is generally hostile to unions, with Gov. Phil Bryant repeatedly expressing opposition. Mississippi Economic Council President Blake Wilson said Nissan shouldn't be obligated to stand aside while the UAW seeks unionisation.

"This is certainly the union's right to come here and try to organise, but it is also the employer's right to not invite the union in," said Wilson, whose group is the state chamber of commerce.