In an election characterized by countless headlines, the refusal of Republicans to support their party's nominee was a constant topic of discussion in 2016. Our paper looks to explain why Republican members of Congress joined the socalled #NeverTrump movement. In the first part, we document the varied-and often contradictory-explanations of the #NeverTrump movement offered by journalists, pundits, and politicians during the campaign. We then categorize these popular explanations into four theoretical categories: policy preferences, identity, electoral motivations, and establishment dynamics. In the second part, we test the varied claims. We believe two findings stand out and have broader implications for American politics. First, despite the popular belief that members of Congress are single-minded in their pursuit of reelection, we found that a lawmaker's religion and sex-both in the identity category-had the largest effects on the decision to join the #NeverTrump movement. Second, the results show that establishment Republicans were more likely to support Donald Trump's candidacy. Notably, the direction of this effect is inconsistent with popular explanations of the #NeverTrump movement but consistent with a range of academic studies.

Donald Trump’s first year in office received unprecedented media coverage, with many wondering whether congressional Republicans were “adversaries” or “allies” of the president’s legislative positions. We explore this issue from two vantage points. First, we place Trump’s presidency in historical context by forecasting his Republican support with data from 1969 to 2016. We find that Republicans supported Trump’s legislative positions in 2017 at levels consistent with expectations, contrary to the views of some. Second, we explore the factors that explain why Republican lawmakers supported or opposed their party’s president. We find that conservative and establishment Republicans were more likely to support Trump, contrary to some claims, while female Republicans and those representing affluent, non-white districts were more likely to oppose Trump. We conclude by discussing the broader implications of these results, including the role of identity in contemporary American politics and the possible realignment of the GOP.