The decision will be no less momentous for President Trump's Justice Department, because a victory for the government could rewrite the way antitrust rules and takeovers are applied. Here's a primer on what is about to happen:
What's the big deal?
AT&T, which most Americans know as a mobile-phone provider, is trying to buy Time Warner, owner of big media brands including HBO, Warner Bros. and CNN.
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The takeover was announced in October 2016, and is the latest effort by a big telecom or cable company to acquire media assets.
The Justice Department sued to block the deal, arguing that it would limit competition and raise costs. The companies have countered that the deal would allow Time Warner and AT&T to compete more effectively against Silicon Valley companies like Google and Netflix.
When will we know the decision?
The case will be decided by Richard J. Leon, a United States District Court judge who was appointed by President George W. Bush. He is expected to give a shortened version of his opinion in remarks around 4 p.m. Eastern time on Tuesday. No devices are allowed in the courtroom, so the decision isn't likely to be known to the public until 4:30 or 5 p.m. The full opinion, released around the same time, could be more than 200 pages.
What are the possible outcomes?
Judge Leon could block the deal.
Doing so might encourage the Justice Department to act more aggressively when it looks at deals in the future, and might prompt a rethink by companies with similar deals in the works.
A key argument against the government's case is that the deal is a so-called vertical merger, which means that the two companies do not produce competing products: One makes media content, and the other distributes it. Some big takeovers lately have had similar profiles — the purchase of the insurer Aetna by the drugstore chain CVS, and Amazon's purchase of Whole Foods — and they typically make it past regulators.
So a win for the government could really shake up some businesses' plans and open the door for a new definition of antitrust regulation.
He could let it proceed without attaching any conditions.
That would, of course, have the opposite effect. It could be the green light for more vertical takeovers, and would be seen as a setback for the Trump administration.
If the deal is allowed to proceed, Comcast is expected to make a bid for most of 21st Century Fox's television assets — setting up a bidding war against the Walt Disney Company, which has already struck a deal to buy those holdings.
It's possible that the Justice Department will appeal a ruling that goes against it, though, so things may not end there.
He could approve it but attach conditions.
The aim of the conditions would broadly be to keep AT&T from using its control of content like HBO or CNN as a weapon to increase costs for its rivals. The fear is that AT&T could charge rivals a high price for, say, HBO to make AT&T's own product more competitive.
One way to address this would be to appoint a third party to oversee disagreements between AT&T and the cable companies that want to license Time Warner content. The government doesn't like that approach.
Another option is to demand that AT&T and Time Warner sell off some plum assets. AT&T and Time Warner don't like this approach, so would be expected to appeal any such decision.
(Disclosure: Comcast is parent of NBCUniversal and CNBC.)