“…Far from being a "cattle prod to agency heads" (quoted in Dawson and Kleiner, 2015: 202), what empirical work exists on the subject suggests that lettermarks from members of Congress alone are not effective in securing particularistic outcomes for individual legislators (Mills and Kalaf-Hughes, 2015;Mills, Kalaf-Hughes, and MacDonald, 2016). But existing evidence on this point is drawn from one federal agency, the Federal Aviation Administration (FAA), and employs one, particularly visible, measure of congressional success: the issuance of a national interest exemption for control towers that were marked for closure (Mills and Kalaf-Hughes, 2015;Mills, Kalaf-Hughes, and MacDonald, 2016). While consistent with a lettermarks "do no harm" or "don't matter" conclusion, the nature of these requests means that they are likely very visible to organized interests, affected constituents, and ultimately, voters.…”