It's been a month since Amazon CEO Jeff Bezos announced that he is buying the upscale Whole Foods Market grocery chain for $13.7 billion, or $42 a share, in an all-cash transaction.

Now, opposition is mounting against the pending purchase. Proposed federal class-action shareholder lawsuits have been lodged to block the deal, arguing that it isn't good for Amazon shareholders. The United Food and Commercial Workers International Union is complaining to the Federal Trade Commission that the accord would reduce competition, limit consumer choice, and kill jobs. And a member of the Subcommittee on Regulatory Reform, Commercial and Antitrust Law is demanding congressional hearings.

"Competition is essential for a healthy economy. That's true across the board. Amazon's proposed purchase of Whole Foods could impact neighborhood grocery stores and hardworking consumers across America," said Rep. David Cicilline, a Democrat of Rhode Island and the committee's ranking member. "Congress has a responsibility to fully scrutinize this merger before it goes ahead. Failing to do so is a disservice to our constituents."

And let's not forget President Donald Trump's campaign statements about Bezos, who also owns the aerospace concern Blue Origin and the Washington Post. "He thinks I'll go after him for antitrust," Trump said at the time. "Because he's got a huge antitrust problem because he's controlling so much; Amazon is controlling so much of what they are doing.”

Trump said Bezos was "using the Washington Post, which is peanuts; he's using that for political purposes to save Amazon in terms of taxes and in terms of antitrust." The then-candidate's campaign staff clarified the statement, saying the newspaper was being harnessed as leverage to protect Amazon from getting "sued for monopolistic tendencies that have led to the destruction of department stores and the retail industry."

Meanwhile, at least one hedge fund manager has said he's shorting Amazon shares out of fear of what Washington might do with the merger.

Douglas Kaas, president of Seabreeze Partners Management, said the following:

I am shorting Amazon today because I have learned that there are currently early discussions and due diligence being considered in the legislative chambers in Washington, D.C., with regard to possible antitrust opposition to Amazon's business practices, pricing strategy, and expansion announcements already made (as well as being aimed at expansion strategies being considered in the future). If I am correct, word of this could lower Amazon's shares by 10% overnight. And if expansion or pricing prohibitions are attacked for antitrust reasons (or for other reasons), a far more severe market impact is possible.

Cicilline, the Rhode Island representative, says the Amazon-Whole Foods transaction "occurs during a long period of economic concentration that has already caused a decline in workers' wages and mobility, essentially allowing a small number of monopolists to hoard the 'fruits of economic growth.'" However, the lawmaker notes that two prominent scholars don't think the deal will increase market power and create higher consumer prices.

A combined Amazon and Whole Foods would own 1.4 percent of the US grocery market, putting it in 10th place behind Walmart, which has a 14.5 percent share, according to CNBC. Barron's notes that "Amazon’s history of disrupting industries and its wide distribution" is adding to "the perceived threat."

"By buying Whole Foods, Amazon will have physical locations within a one-hour drive of 70% of U.S. households, meaning the company will be able to deliver items more quickly than ever," Barron's said.

The US government has been mum so far on whether it will take any action to scuttle the deal, which has not yet been approved by Whole Foods shareholders. That said, the stock price of Whole Foods is hovering pennies below the $42 purchase price, which suggests shareholders are all-in on the deal—at least for now.