Senator Bernie Sanders joined the drive Friday to unionize Amazon workers in Alabama as clashes intensified between lawmakers and the e-commerce giant ahead of a deadline for a vote that could lead to the first union on US soil at the massive tech company.

The visit by Sanders, joined by actor Danny Glover and rapper "Killer Mike" Render, marks the latest high-profile appearances in the contentious organizing effort for some 5,800 employees at Amazon's warehouse in Bessemer which culminates next week.

"If Amazon workers in Alabama can prevail in forming a union, it can be done all over this country," said Sanders, a former presidential candidate with a loyal following among progressives.

The months-long drive led by the Retail, Wholesale and Department Store Union has sparked national attention and intense debate over workplace conditions at Amazon, which has more than 800,000 US employees.

Amazon has argued that most of its workers don't want or need a union, and that it already provides more than most other employers, with a minimum $15 hourly wage and other benefits.

Dave Clark, head of Amazon's consumer business, said the company delivers on the goals of Sanders.

"I often say we are the Bernie Sanders of employers, but that's not quite right because we actually deliver a progressive workplace," Clark tweeted.

But unions and political leaders have argued that Amazon employees face constant pressure and monitoring, with little job protection, highlighting the need for collective bargaining.

RWDSU president Stuart Appelbaum said the struggle is "about the unregulated technologies Amazon uses to monitor every movement of its workers, including the amount of time they spend in the restroom... and, most importantly, about the lack of dignity, respect and just treatment too many Americans experience at work. "

Sanders and others say the Amazon battle highlights growing economic inequality at a time when billionaires, such as Amazon founder Jeff Bezos, have seen their wealth increase during the pandemic.

"All I want to know is why the richest man in the world, Jeff Bezos, is spending millions trying to prevent workers from organizing a union so they can negotiate for better wages, benefits and working conditions," Sanders tweeted ahead of his visit.

Signs outside the Amazon fulfillment center BHM1 in Bessemer, Alabama, during a unionization drive on March 5
Signs outside the Amazon fulfillment center BHM1 in Bessemer, Alabama, during a unionization drive on March 5 GETTY IMAGES NORTH AMERICA / Megan Varner

Speaking to reporters after addressing Amazon workers, Sanders said he gave Amazon credit for raising its minimum wage to $15 an hour.

"But right now, they are nervous that if workers here are successful it will spread all across this country," he said.

"But everybody here should know that, in Europe, Amazon deals with unions. So they sure as hell can deal with union in the USA," Sanders said.

Tensions have escalated as Amazon has fired back against criticism from lawmakers and others.

"If you want to hear about $15 an hour and health care, Senator Sanders will be speaking downtown," Clark tweeted, while pointing out that in Sanders state of Vermont, the minimum hourly wage is just $11.75.

"But if you would like to make at least $15 an hour and have good health care, Amazon is hiring."

Representative Mark Pocan of Wisconsin responded to Amazon that "Paying workers $15/hr doesn't make you a 'progressive workplace' when you union-bust & make workers urinate in water bottles."

Amazon fired back: "You don't really believe the peeing in bottles thing, do you? If that were true, nobody would work for us."

The tech giant meanwhile escalated a feud with Senator Elizabeth Warren, who accused Amazon of avoiding taxes.

"You make the tax laws @SenWarren; we just follow them," Amazon said on Twitter. "Amazon has paid billions of dollars in corporate taxes over the past few years alone."

Warren hit back with a barbed tweet at the company.

"I didn't write the loopholes you exploit, @amazon - your armies of lawyers and lobbyists did," she wrote.

"But you bet I'll fight to make you pay your fair share. And fight your union-busting. And fight to break up Big Tech so you're not powerful enough to heckle senators with snotty tweets."