AT&T is apparently readying a Hail Mary pass to save its foundering $39 billion bid for T-Mobile. Trouble is, there's every reason to think it won't be enough.

According to Bloomberg, the telecom giant is preparing to offer a deal to the Department of Justice under which AT&T would divest as much as 40 percent of T-Mobile's assets as part of the acquisition. It's not at all clear which parts of T-Mobile AT&T might consider shedding as part of this plan.

The Justice Department filed suit to block the deal almost three months ago, saying the combination of AT&T (the second-largest U.S. wireless carrier) and T-Mobile (the fourth) would "substantially lessen competition" in the mobile market. AT&T's divestiture plan aims to address DOJ's concern by selling a big enough chunk of T-Mobile's business to rivals in order to preserve wireless competition in the U.S. At least in theory, that is.

The divestiture plan carries more than a whiff of desperation, given rapidly mounting opposition to the deal. Earlier this week, FCC chairman Julius Genachowski came out in opposition to the merger and asked other FCC commissioners to refer the case to an administrative law judge--a step that would prolong the acquisition and add to AT&T's costs, if it didn't scuttle the deal entirely.

In response, AT&T on Wednesday said it would take a $4 billion charge in the fourth quarter to cover a breakup fee to T-Mobile should the deal collapse. AT&T also withdrew its application for FCC approval of the deal, saying it wanted to focus on the Justice Department case first.

Analysts Bloomberg interviewed, however, were skeptical that divestiture will do much to sway the feds. Kevin Smithen, an analyst with Macquarie Securities USA Inc. in New York, called AT&T's plan "problematic," noting that there aren't very many potential buyers for parts of T-Mobile. And the Justice Department isn't likely to smile on the notion of, say, U.S. wireless leader Verizon acquiring a big piece of T-Mobile.

What's more, federal officials appear ready to argue that even a 40 percent divestiture wouldn't alleviate their antitrust concerns. On a conference call this week, officials said that an AT&T takeover of T-Mobile would harm competition in 99 of the top 100 markets in the U.S. They also scoffed at AT&T's claims that the merger will help it roll out new 4G LTE services faster and create jobs.