Looking at the 2004 numbers, it should not be an insurmountable number. GM 27.3%, Toyota 12.2%, or a total of just under 40% of the US market. They could try to spin off a couple of lines to lower the percentage if they had to - to put a little dressing on the deal. Given the historical Republican support for big business, the deal should pass antitrust muster.
It would be unlikely to be a straight up merger, as Toyota would not want to assume the pension and health care liabilities.
I wonder if Toyota would want to own a piece of the UAW. I doubt it, and the UAW is not going to just let them have GM.
GM and Ford were both foolish not to buy the Japanese out during the Japanese depression. Of course, I'm not sure that the Japanese would have let them do that anyway.