I don't think the DOJ will approve one or two and not the others. There is little overlap with the AMR/NWAC merger, only 11 common routes, the hubs at MSP, ORD, and DTW won't be any different than PHI, PIT, and BWI in terms of geography. The DOJ doesn't consider fleet types in granting their approval and NW and AA do fit well with networks, NW strong in Asia, Pacific NW, AA in S. America, Southeast, and East, and Europe. Also the combined AA/NW would only have 31% of the market before divestures, bigger than UA/US, but not by much. I'm not sure of DL/CO but without a doubt something will have to be done with their networks.
Who knows what is down the road, but I don't think we will see a UA/US without an AA/NW or DL/CO approved by DOJ, other matters like labor could tell a different story.
Legalman, I do think Bush would lean more favorably towards these mergers than Gore.