It's not the administration, DOT or DOJ who will have the ultimate say. It's the courts. There will be lawsuits by consumer groups or other entities no matter which airlines pursue mergers. The AA/B6 Northeast Alliance was essentially scuttled by a court. That's not to say a merger wouldn't make its way to SCOTUS, but I'm not aware of any merger objections getting past the District Court level. Despite he who thinks he's in-charge and can heal all as a "Red Cross doctor" the courts will look at any tie-ups and have the final say.