By Scott Wallsten (March 6, 2018, 3:24 PM EST) -- Evaluating a proposed merger is necessarily imprecise because it requires predicting its future effects. Economic theory by itself provides little guidance since it generally supports arguments that a merger might benefit or harm consumers. Instead, theory guides us to the questions we should try to answer empirically in the context of the relevant facts. The AT&T-Time Warner merger debate is different from most large mergers only in that politics have entered the discussion in a way that has happened rarely in the past. In particular, President Donald Trump has made no secret of his dislike of Time Warner property CNN and publicly opposed the merger. Despite the president's disapproval, the proposed transaction is a textbook vertical merger and should be evaluated as such....