In a statement aimed at Amazon, US President Joe Biden has warned companies against intimidating staff considering joining a labour union.
Mr Biden said "the choice to join a union should be up to the workers - full stop".
His remarks come in the middle of a historic vote in Alabama, where Amazon warehouse workers are deciding whether to join a union.
It is the first such vote Amazon has faced in the US since 2014.
It follows months of criticism of the e-commerce giant for falling short of coronavirus safety precautions, while making high demands on workers during the pandemic, when its business has boomed.
In his video remarks, Mr Biden did not explicitly back the union effort, nor did he mention Amazon by name. However, he said the White House was committed to the right to collectively organise.
Workers in Alabama – and all across America – are voting on whether to organize a union in their workplace. It’s a vitally important choice – one that should be made without intimidation or threats by employers.— President Biden (@POTUS) March 1, 2021
Every worker should have a free and fair choice to join a union. pic.twitter.com/2lzbyyii1g
"Today and over the next few days and weeks workers in Alabama - and all across America - are voting on whether to organise a union in their workplace. This is vitally important - a vitally important choice," he said.
"There should be no intimidation, no coercion, no threats, no anti-union propaganda. No supervisor should confront employees about their preferences."
Amazon, which has been accused of taking aggressive steps to quash labour activism, did not respond to a request for comment.
It has previously described its response - which staff and union organisers have said include mandatory meetings and anti-union fliers throughout the warehouse, among other measures - as providing "education, about a step it says could lead union members being charged $500 in dues without a guarantee of better pay or other benefits.
High-profile union fight
Nearly 6,000 people work at the Amazon warehouse in Bessemer, Alabama, where the organising drive is taking place. Mail-in ballots are due by 29 March.
If organisers are successful, it will be Amazon's first unionised workplace in the US.
The fight has attracted support from many national Democratic heavyweights, including former presidential candidate Senator Bernie Sanders. Mr Biden, who was heavily backed by unions in his presidential campaign, has cast himself as a friend to organised labour.
Republicans, who tend to oppose organised labour, have largely steered clear of the fight - unlike other high-profile union drives, when they sided with employers.
Stuart Appelbaum, the president of the Retail, Wholesale and Department Store Union, which is leading the effort in Alabama, thanked Mr Biden for what he said was a "clear message of support".
"As President Biden points out, the best way for working people to protect themselves and their families is by organising into unions. And that is why so many working women and men are fighting for a union at the Amazon facility in Bessemer, Alabama," he said.