The union representing journalists at the Bloomberg Industry Group demanded management remove its ban on investigating Michael Bloomberg and his 2020 Democratic rivals in a letter published Monday night.

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"We are extremely alarmed by management's decision to silence the journalists we represent at Bloomberg Industry Group, as well as the unrepresented journalists at Bloomberg News," the union wrote. "We call on Bloomberg corporate management to rescind its policy and allow journalists throughout the Bloomberg family to do their jobs."

The ban on investigating the 2020 Democratic field dumbfounded members of the media when Bloomberg editor-in-chief John Micklethwait announced it Sunday. Bloomberg employs 2,700 journalists and analysts.

"The Guild believes journalists should not only be allowed - but encouraged - to thoroughly cover every single candidate as the profession demands in one of the most important elections in modern history," the union wrote. "A journalist's job is, above all, to cover the news and to serve as an independent monitor of power."

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A spokesman for Bloomberg Industry Group referred FOX Business to a statement by Cesca Antonelli, the group's editor-in-chief.

"For nine decades, our subscribers have relied on us to cover the policy implications of elections at the state and national level – and that's our plan for 2020," Antonelli said. "As we planned long before Mike entered the race, our focus will be principally on the key congressional and gubernatorial races impacting legal, tax, and government professionals."

Members of the media from both sides of the aisle criticized the ban.

"I am a paid subscriber to Bloomberg News because there are so many great journalists there doing amazing work but this is a real disservice to all of them," journalist Judd Legum wrote on Twitter. "You can't declare the biggest story of the next 12 months off limits."

"Is this an early April fools joke? This decision makes a mockery of journalism. Their bosses actually ordered them to be fake news!" Media Research Center founder Brent Bozell wrote on Twitter.

"As a journalist, watching what's playing out at Bloomberg News is making me sick to my stomach. Mike Bloomberg once promised he'd sell if he ran for president. In not caring about a) conflict of interest or b) journalistic integrity, Mike Bloomberg is exactly like Donald Trump," Will Bunch of the Philadelphia Inquirer wrote on Twitter.

The entry of Bloomberg into the presidential race also raises potential conflict-of-interest questions involving his extensive business holdings, which go well beyond his news service. Bloomberg's businesses, which include selling financial data services, employ more than 19,000 people in 69 countries.

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The Associated Press contributed to this report.