More than a decade after George W. Bush promised to “change the tone in Washington” and three years since Barack Obama pledged to “end the politics of division,” Congress is even more polarized. Major legislation, including reform of the health-care and financial sectors and the economic stimulus bill, has been enacted on partisan votes. The pattern of retirements and nominations suggest that the next Congress may be even more polarized along party lines than the current one.

Many observers have sought to account for this seemingly inexorable trend, often focusing on particular factors, such as redistricting or the realignment of voters. Sean Theriault notes that his book “will not satisfy the reader who is in search of the cause” (p. 9) of party polarization in Congress. Rather, his goal is to present a comprehensive and integrated account of polarization. He succeeds in his objective, presenting a balanced and readable study in which summaries of statistical tests are enlivened by vignettes about members of Congress.

Theriault explains that he is “building upon the existing polarization studies” (p. 10) and does so effectively. He uses Keith Poole and Howard Rosenthal's NOMINATE scores to measure polarization. He points to the “sorting” of citizens into more ideologically cohesive party coalitions noted by Morris Fiorina, along with the geographic clustering of like-minded Americans found by Robert Cushing and Bill Bishop. In noting the contribution of “procedural” votes to polarization, Theriault agrees with Frances Lee and Robert Van Houweling. His claim that constituency changes that yield more cohesive party caucuses empower leaders who exacerbate polarization recalls John Aldrich and David Rohde's theory of “conditional party government.” These acknowledged debts notwithstanding, Theriault offers readers many original analyses and makes his own contribution to this maturing literature. His judgments rest upon multiple tests, making his basic claims more persuasive.

Polarization occurs via a combination of replacement and behavioral change on the part of senators and representatives. Newer cohorts of legislators are more polarized than Capitol Hill veterans, but long-serving members have also increasingly voted with their parties. Theriault concludes that turnover's contribution to polarization is about twice as great as that of conversion. He also notes that much of the polarization has occurred outside the South.

The causes of polarization in this study can be grouped in two broad categories: constituency factors and congressional practices. The constituency factors discussed are numerous. While acknowledging that redistricting's effect has been exaggerated in the media, Theriault does see it contributing modestly to polarization, by producing districts that are slightly less competitive. Legislators in safe seats have less incentive to take middle-of-the-road stands. Geographic sorting of Americans has had the same effect, resulting in the famous “red and blue” map. Split-ticket voting has declined, leaving fewer legislators cross-pressured between their presidents and their districts. Ideological self-sorting of liberals and conservatives into party coalitions has meant that the electorates faced by members of Congress in primaries are less and less friendly to moderates. Finally, using convention delegate studies, the author shows that GOP and Democratic activists are increasingly polarized. Militant activists are one more force favoring the nomination of candidates who vote the party line.

Yet while these constituency factors are widely discussed, Theriault argues that they can directly explain less than one-third of the polarization that has occurred. Party-line voting has increased substantially among legislators representing competitive districts as well as those enjoying safe seats. Accordingly, to fully account for the trend we must look to developments on Capitol Hill. As parties have become more cohesive, they have chosen more polarizing leaders who have adopted tactics that deepen the partisan divide. House leaders have relied upon restrictive rules, allowing few, if any, amendments to be offered when bills reach the floor. These tactics increase polarization since amendments offered by the minority might otherwise peel off some members of the majority. In the absence of amendments, the highly partisan procedural votes make up a larger share of all votes, increasing polarization in the aggregate.

Theriault shows that measures of constituency are more predictive of members' votes on final passage of bills than on procedural questions such as the rule and the motion to recommit, which are obscure to voters and on which party affiliation tends to trump all. Legislators' propensity to side with their party on procedural votes is not new, but it has grown. Similarly, increasing use of the filibuster in the Senate has made cloture votes more common.

These factors are not wholly independent. Increasingly cohesive party caucuses stem in part from more polarized constituencies. Yet these party caucuses also increasingly elect leaders whose actions contribute to polarization in Congress. The ensuing clashes sharpen the images of the parties, sending signals to voters and increasing the correlation between ideology and party affiliation at the mass level. The more polarized primary electorates and activist cadres that emerge are then more likely to nominate party-liners than moderates in what Theriault terms a “vicious cycle.”

The Senate as well as the House has polarized along party lines. The Senate remains marginally less polarized than the House, a gap that has persisted for decades, even as party divides have grown in both chambers. Theriault notes that many of today's most conservative Republican senators served in the House after Newt Gingrich began to reshape his party and contends that they brought the confrontational attitudes and practices he fostered there to the Senate.

The author sticks close to his roll call and constituency data, and is forthright about their limitations. For example, he notes that they cannot reveal whether the parties are growing farther apart on substance or simply becoming more cohesive teams. Indeed, contrasting recent divides over Iraq, health-care reform and gay rights with earlier struggles over Vietnam, the creation of Medicare/Medicaid and Civil Rights for African Americans, it is doubtful that there is more at stake today than there was at the nadir of partisan polarization in the middle and late 1960s.

There are other topics that Theriault might have explored further. He finds that Republican legislators, unlike Democrats, are increasingly responsive to their party's activists. GOP representatives are also far more likely than Democrats to deprive moderates of committee chairs that they would otherwise inherit on the basis of seniority. (After this book was published House Democrats also abandoned the term limits on committee chairs that their GOP counterparts had imposed in 1995. In the Senate, Democrats had never limited chairs' terms the way Republicans did.) Former House Republicans, but not House Democrats, have helped polarize the Senate. These important asymmetries are noted but largely unexplained in this study.

Another factor meriting greater attention is the role of legislators' own preferences in accounting for polarization. Members of Congress were voters and, often, party activists long before they were elected. They, too, have convictions. Might not the same signals that earlier rounds of polarization sent to voters and activists also produce a self-sorting among potential candidates, with conservatives gravitating to the GOP, liberals moving toward the Democrats, and moderates now less likely to seek nomination by either party?

The fact that Theriault leaves these questions to other scholars does not diminish the contribution he makes in this timely and well-crafted volume. Future authors exploring polarization will take this book as a point of departure.