A decision is expected Tuesday afternoon in the government's lawsuit to block AT&T's purchase of Time Warner.
Shares of both companies were rising in early trading Tuesday.
The so-called "vertical merger" would give AT&T the ability to distribute more Time Warner content including HBO and CNN.
"Everything's on the line for the Department of Justice," a former antitrust official said.
Ahead of the ruling, AT&T was up about 1% in early trading while Time Warner was higher by about 0.9%.
The merger, if permitted by Judge Richard Leon, is widely considered to be a harbinger of future media mergers that could radically shift how Americans consume television and movies going forward. Most notably, Disney's ongoing offer to buy 21st Century Fox for $52.4 billion, which Comcast is expected to compete against.
In its suit against AT&T, the Department of Justice alleges the merger would "greatly harm American consumers" through "higher monthly bills and fewer of the new, emerging innovative options that consumers are beginning to enjoy," Makan Delrahim, head of the Justice Department's antitrust division, said when announcing the suit last year.
President Donald Trump voiced opposition to the merger back in November on the day after the suit was announced by his administration. He said the deal is "not good for the country," echoing similar statements he made on the campaign trail throughout 2016.
AT&T rebuffed those claims made by the Justice Department and Trump, saying similar mergers are "routinely approved because they benefit consumers without removing any competitor from the market."
The merger is considered "vertical" because it would combine Time Warner, which makes programming, with AT&T, a distributor of that content, which would then have access to more Time Warner brands like HBO, TNT, TBS, and CNN.
"Everything's on the line now for the Department of Justice," Gene Kimmelman, a former DOJ antitrust official who now runs a consumer advocacy group, told the Washington Post. "They either come out as enormous victors … or they’ll face an avalanche of new transactions if they lose this case."
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