(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.

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.

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