(CNN) -- Amazon just raised its minimum wage to $15, but that's not enough for some progressive politicians.
Sens. Elizabeth Warren and Bernie Sanders sent a letter to Amazon CEO Jeff Bezos on Tuesday scolding the company for allegedly distributing a 45-minute instructional video to managers at recently-acquired grocery chain Whole Foods on how to defuse union organizing.
"Workers' rights do not stop at the minimum wage, and raising the pay of your lowest-paid workers, while important, does not give you a free pass to engage in potentially illegal anti-union behavior," Massachusetts Democrat Warren and Vermont independent Sanders wrote.
Amazon did not immediately return a request for comment.
The letter comes as Warren prepares for an all-but-certain bid for the 2020 Democratic presidential nomination. She has ramped up travel to early-voting states and told a crowd at a Massachusetts town hall two weeks ago that after the midterms she will "take a hard look at running for president."
Warren this week released the results of a DNA test intended to combat President Donald Trump labeling her "Pocahontas" over Warren being listed in 1980s and 1990s law school faculty handbooks as Native American.
Her political team has also turned Senate Majority Leader Mitch McConnell's 2017 admonishment that Warren was warned against reading a letter from Coretta Scott King criticizing Jeff Sessions on the Senate floor, but "nevertheless, she persisted," into a slogan. It handed out printed "PERSIST" signs at Netroots Nation, a major progressive gathering, in New Orleans in August.
The aggressive moves are intended to make Warren's intentions clear to progressives -- and demonstrate that she is capable of fighting powerful Republicans — as Democrats prepare for a wide-open presidential nominating contest that more than two dozen mayors, governors, senators and House members are considering entering.
In announcing its $15 minimum wage, Amazon said it had "listened to our critics." Most prominent among them was former Democratic presidential candidate Sanders, who introduced legislation aimed squarely at charging the Seattle e-commerce giant for any safety net benefits its employees used.
The letter to Bezos refers to a videooriginally reported by Gizmodo in early September that allegedly told team leaders how to recognize signs of unrest among workers, and provided arguments for why a union would not be in the interests of the company or its workforce.
"Our business model is built upon speed, innovation, and customer obsession—things that are generally not associated with unions," the video said, according to Gizmodo. "When we lose sight of those critical focus areas we jeopardize everyone's job security: yours, mine, and the associates'."
The senators' letter raised concerns that, if genuine, the video would constitute violations of the National Labor Relations Act, the law that protects worker organizing. Specifically, suggestions that a facility might close down if employees organize and any attempt to spy on union activity could be grounds for a complaint to the National Labor Relations Board.
No recent charges appear on the Board's web site, and a call to the Board was not immediately returned.
The senators requested the full video distributed by Amazon to Whole Foods managers, any other materials pertaining to organizing activities and a list of law firms and consultants the company may have retained to help tamp down labor unrest.
The Wall Street Journal reported in September that a nascent union organizing campaign was underway at Whole Foods.
Unions are relatively rare in the industry, representing only 5% of retail workers in 2017, according to the Bureau of Labor Statistics.
Warren, long a critic of large banks and corporations, also sent a letter Tuesday taking hedge funds to task for their roles in the bankruptcy of Toys 'R' Us, which resulted in the loss of 33,000 jobs.
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