The Senate is a difficult institution for social scientists: Its reliance on norms, its individualistic rules, and the outsized personalities of many of its members make the chamber resistant to causal analysis. Peter Hanson is one of the few scholars to have successfully tackled this peculiar, fascinating, and often frustrating legislative body. In Too Weak to Govern, he proposes a persuasive new theory of Senate majority party power that explains the conditions under which party leaders circumvent the regular appropriations process and consolidate spending bills into single legislative packages (omnibus bills or continuing resolutions), often against the will of other senators.

Hanson’s “limited influence” theory posits that Senate majority party leaders assert themselves in appropriations politics when their parties are “weak,” meaning small, divided, and ideologically distant from the minority party (p. xvi). The logic is that members of the majority ordinarily prefer the “regular order”—considering individual appropriations bills separately—because they can freely debate and offer amendments to them. When their party is smaller and less unified, however, that increases the likelihood of politically embarrassing amendments being proposed, debated at length, and possibly even adopted. Under those conditions, majority party “members are more likely to accept the limits omnibus bills place on their ability to participate in lawmaking” (p. 35).

Limited influence theory strikes a middle ground between paradigms of powerless Senate majority parties and models of highly influential ones. It is also a dramatic contrast to theories of the House of Representatives that depict the majority party as a powerful association (cartel theory) or led by stronger leaders when the party is in greater ideological agreement (conditional party government theory). The reason for this contrast, Hanson writes, is that the Senate’s rules mean that the majority party “cannot systematically advance a partisan policy agenda,” nor does it “dominate the floor as the majority party can in the House,” but it does have “a limited ability to control the agenda that is used to stave off defeat and avoid policy harm when it is threatened” (p. 11).

Hanson assesses his theory with both quantitative and qualitative data. In Chapter 2, he uses a series of regression analyses to demonstrate that between 1975 and 2012, a majority party that is smaller, less unified, or more ideologically distant from the minority has been more likely to use continuing resolutions or omnibus spending bills than to consider appropriations bills individually. Importantly, he fails to find a similar dynamic in the House of Representatives.

Although the findings in this chapter are intriguing, to me they are somewhat less persuasive. The statistical relationships that Hanson finds are not terribly strong, there are problems of multicollinearity, and he fails to control for the homogeneity of the minority party, which would seem to be an important predictor of the likelihood that minority amendments will succeed on the floor. Furthermore, regression tests cannot capture aspects of the appropriations process that are likely to be causally important, such as the extent to which the minority is expected to file multiple amendments on a bill.

To his credit, however, Hanson not only readily acknowledges the limitations of quantitative analysis but also addresses most of them by dedicating his next three chapters to case study analysis. Each one details the appropriations process in the Senate between 1981 and 2004, with particular attention to the reasons that individual spending bills either became part of omnibus measures or were replaced by continuing resolutions. Chapter 3 discusses the “first wave” of irregular order on appropriations bills, characterized by the struggles of majority party Republicans (1979–87); Chapter 4 reviews the return (more or less) to regular order (1987–95); and Chapter 5 examines the current era (1996– ) in which heightened party conflict has hampered the ability of both parties to follow regular order when in the majority.

Nuanced, well-researched, and engagingly written, the case study chapters allow Hanson to tease out key political factors that lead the Senate majority party to resort to (or avoid) omnibus bills or continuing resolutions. These include the insertion of particularly controversial matters into debates on appropriations bills, like school prayer and abortion; the role of individual senators in derailing the regular order; whether objections to spending bills come from within the majority party (as was often the case in the 1980s) or the minority party (as happens today); and the complex and shifting relationship of the Senate, presidential politics, and the House’s consideration of appropriations measures.

The case study chapters are very strong, though they do suffer from a couple of minor omissions. Hanson gives short shrift to Appropriations Committee politics in his case studies, yet one would expect that it is in the committee where troublesome amendments first make an appearance and where voting patterns serve as signals to other senators about the partisan nature of a spending bill. The author also says relatively little about the internal Senate dynamics that led the chamber to move to regular order after 1986, though—as Hanson rightly points out—there is “an inherent difficulty of explaining why something did not happen” (p. 105).

Empirical evidence aside, Hanson’s theory makes intuitive sense. It takes into account the “must pass” nature of spending bills, which means that the power of the majority leader increases as the expiration of each fiscal year approaches and his copartisans fear the collective harm of failing to fund the government. It also recognizes the underappreciated ability of majority party leaders to innovate procedurally. Nonetheless, I did take exception to the theory’s assumption that reelection is what principally motivates the majority, because omnibus bills and continuing resolutions also allow the majority party to shoehorn desired policy proposals into bills and limit unwanted policy changes via amendments, thereby helping the party achieve collective policy goals. Hanson counters this argument by observing that aggregate spending bills tend to pass by bipartisan votes, but a final passage vote is not by itself the most reliable indicator of the ways in which parties view the policy contents of a bill, particularly large and complex ones.

Hanson distinguishes limited influence theory from other models of legislative party power. But as he notes, two of the most prominent (cartel theory and conditional party government theory) best fit the House of Representatives, not the Senate. Also, much of the “weak party” literature on the Senate predates the chamber’s heightened partisanship and the growing influence of party leaders. While Hanson’s theory grants majority parties less influence than Nathan Monroe and Chris Den Hartog’s “costly consideration” model, the author looks solely at appropriations bills, making an apples-to-apples comparison difficult. This underscores the paucity of good theories about party power in today’s Senate and why Hanson’s study is thus so valuable.

In short, Too Weak to Govern is a worthy contribution to our understanding of the U.S. Senate. It not only shows that majority parties in the chamber can exercise influence in the appropriations process but also shows the conditions under which they do. Those who maintain that Senate parties and their leaders are without power or, conversely, that they are effectively the same as their counterparts in the House will have to reckon with Hanson’s fine tome.