The top executive for health insurer Aetna essentially threatened to leave Obamacare if the federal government tried to block its merger with Humana.

In a letter obtained by the Huffington Post, Aetna CEO Mark Bertolini wrote last month to the Justice Department that his company would experience a "negative financial impact" if the DOJ were to try to block the proposed merger, making it difficult to continue selling Obamacare plans.

While some Democrats had already accused Aetna of leaving many of the Affordable Care Act's marketplaces as retribution against the DOJ's attempt to block the merger, the letter sheds additional light on why Aetna made its decision.

Bertolini wrote that his company has been losing money in the insurance marketplaces ever since they launched in 2014, and indicated that it needs to be allowed to merge with Humana in order to move forward on a strong financial footing.

"Our ability to withstand these losses is dependent on our achieving anticipated synergies in the Humana acquisition," Bertolini wrote.

Aetna announced Monday that it will no longer sell insurance plans on all but four of the state marketplaces set up under the Affordable Care Act, making it the third major insurer to step back and promulgating fears that the marketplaces won't be an attractive place for consumers.

Last month, the DOJ announced it's suing to block two big proposed insurance mergers, including the merger between Aetna and Humana, as it would reduce the number of large insurance carriers from five down to three.