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Long-haul buses between major Mexican and US cities are feeling the pinch of the ULCC airlines like Volaris, I'm not sure if there are any direct lines at the moment.
Alternatively you could ride from DF over to Guadalajara on Primera Plus, Elite or some other line. From Guadalajara there there are several companies that go straight up to LA:
Otherwise there are various bus lines to both DF and LA from other places in Central Mexico like Durango.