That's the basic issue, however, since the combination of the two operators involves two large accumulations of licensed spectrum and markets, the FCC very much has their say in whether and how such a deal may be allowed. The DOJ and FCC often work together... they always consult each other in major acquisitions like this.
The mobile operators are licensed/sold a monopoly over use of airwaves. Over the past ~30 years of mobile wireless, the FCC has attempted to create diversity of competition that is fair and open. That is very difficult to do because the cost of building nationwide networks and routinely upgrading them to next generation technologies combined with the massive marketing costs and access to the newest/Hottest devices that drive customers to ops services favors the size and coverage. The tendency of most industries is for the big fish to swallow up the smaller ones. Where a monopoly on basic inputs is established, (think steel, rail or oil barons and old AT&T that were broken up to free up markets), one or two dominant companies tend to form. The problem with that is that over time they benefit from acting like what they have become, sole source monopolists... they become resistant to changes in technology, market evolution, innovative combination of new things... because they can charge more for less and being innovators takes more effort.... why do that when you have it all locked up?
The problem is that wireless is becoming the central requirement for all things digital. That puts the mobile/ICT operators in a position to do what companies are supposed to do - be self interested in making as much money as possible. That is OK so long as there is vibrant competition to keep the natural checks and balances in play. However, that gets back to the fact that the public owns the spectrum to which monopoly rights are sold to mobile operators... thus its up to the DOJ to keep the markets open and competitive.
I'm not sure if they could pull that off or not. It'd give Sprint a controlling interest in their nearest competitor, and it'd be hard to prove it wasn't an anti-competitive move. Not to mention if they did do that and kept both brands operating they'd essentially be competing with themselves.
It would be as much a Department of Justice issue as it is FCC. Sorry scarbrobill. They can try to proceed by DOJ has hinted they would attempt to block the deal. The could take on the DOJ but it could prove to be a poor fight to pick.