Norm Eisen, the chief White House ethics lawyer under former President Obama, warned President Trump on Saturday that his latest tweet targeting CNN could be used against his Justice Department in its legal fight to block AT&T's merger with Time Warner.

"Dear Dodo-in-Chief: u realize that this tweet is going 2be an exhibit against u in the DOJ -TimeWarner antitrust case right? It also smacks of collusion since u did it on day when Vlad cracked down on US networks," Eisen tweeted. "For that reason, it may open u to other civil liability."

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Trump reignited his feud with CNN on Saturday, accusing the network on Twitter of spreading "fake news" and representing the U.S. "poorly" across the globe.

That tweet came days after the Justice Department sued to stop AT&T's $85 billion takeover of Time Warner, the parent company of CNN. The lawsuit argues that the merger violates U.S. antitrust laws and would ultimately hurt consumers by creating a media behemoth.

But Eisen suggested that Trump's tweet could be used to bolster the argument that the Justice Department's lawsuit is rooted in the president's personal dislike of CNN, rather than in genuine concern about antitrust laws.

Eisen also suggested that the president's tweet signals "collusion" between Trump and Russian President Vladimir Putin, who signed amendments into law on Saturday requiring foreign media outlets working in Russia to be listed as foreign agents.

Special counsel Robert Mueller is investigating Russia's role in the 2016 presidential election, particularly whether Trump's campaign coordinated with Russian officials or representatives during the race. Trump has denied any improper contacts or exchanges between his campaign and Moscow.