[Editor's note: Mark P. Jones, chairman of Rice University's political science department, has developed rankings of Texas state senators based on their votes during the regular and special sessions this year. According to his analysis, the most conservative senator was Brian Birdwell, R-Granbury, while the most liberal was Rodney Ellis, D-Houston. Bob Deuell, R-Greenville, and Carlos Uresti, D-San Antonio, were in the middle. But the distance between those two is greater than the distance between the most and least conservative Republicans and the most and least liberal Democrats. Jones' paper follows, along with his charts and rankings of the members of the Senate.]
For decades, political scientists have employed roll call votes to rank members of the U.S. Congress on a liberal-conservative range. The Texas Tribune published my comparable analysis of the Texas House of Representatives in July 2011.
Here, drawing on all 684 non-lopsided roll call votes (i.e., at least two senators on the losing side) held during the combined 2011 regular and first-called legislative sessions, I have ranked the 31 members of the Texas Senate using a Bayesian estimation procedure (IDEAL) developed by Stanford University professor Simon Jackman.
In addition to plotting each senator's specific location, ranging here from the liberal extreme of -1.46 to the conservative extreme of 0.54, I also include the 95 percent credible interval (CI) for this point estimate. Only when a senator's CI does not overlap with that of another senator can we say with any real certainty that their respective locations on the liberal-conservative dimension are credibly distinct. The estimate for every senator's Lib-Con Score as well as the 95 percent CI surrounding it is provided in the chart below, as is a listing of each senator's respective Lib-Con Score and 2011 Liberal-Conservative ranking (from most liberal to most conservative).
We can also use this data to determine the extent to which they are significantly more or less liberal or conservative within their own parties in the Senate. It is very important, however, to keep in mind that these comparisons are strictly focused on intra-party dynamics, and that, for instance, for Republicans the term "moderate" does not necessarily signify a senator is not conservative, only that they have a voting record on the Senate floor that is noticeably less conservative than a substantial number of their Republican colleagues.
The most noteworthy feature of this data is the vast gulf that separates the Democratic and Republican parties in the Texas Senate (a chasm also present in the Texas House). Not only does no Democrat overlap with any Republican, the distance between the most conservative Democrat and the least conservative Republican is greater than both the distance between the most liberal and most conservative Democrat and the least conservative and most conservative Republican. However much the members of the delegation squabble over matters among themselves, partisanship remains the greatest divide in evidence.
The 19 Republican senators (in red in the graphic) can be placed into four rough (albeit somewhat arbitrary) groups based on their Lib-Con Score and its distance from those of their colleagues.
The first group consists of five senators who occupy the moderate end of the Republican ideological continuum in the Senate, with Lib-Con Scores noticeably less conservative than an absolute majority of their colleagues (with one exception). This group includes Bob Deuell of Greenville (who is significantly less conservative than 13 Republican senators), Robert Duncan of Lubbock, Kel Seliger of Amarillo, Kevin Eltife of Tyler and John Carona of Dallas (the one exception).
The second and third groups represent the core of the Republican bloc in the Senate, with a dividing line that is not especially clear-cut. The second group includes Craig Estes of Wichita Falls, Steve Ogden of Bryan, Tommy Williams of The Woodlands, Mike Jackson of La Porte, Joan Huffman of Houston, Glenn Hegar of Katy and Jeff Wentworth of San Antonio, who are generally only significantly less conservative and more conservative than a handful of their fellow Republicans, a fact graphically detailed by the overlap between their CIs and those of most other Republicans.
The third group contains all but two of the Senate's most conservative senators. All of the members of this group (Troy Fraser of Horseshoe Bay, Chris Harris of Arlington, Florence Shapiro of Plano, Robert Nichols of Jacksonville and Jane Nelson of Lewisville) are noticeably more conservative than each one of the five members of the first (moderate) group, and in the case of Nelson, than half of her fellow Republicans.
Based on their roll call vote behavior, Brian Birdwell of Granbury and Dan Patrick of Houston are without question the two most conservative members of the Texas Senate. On the liberal-conservative dimension, Birdwell and Patrick form a group unto themselves at the outer reaches of the ideological spectrum, substantially more conservative than over two-thirds of their fellow Republicans, not to mention all 12 Democrats.
The 12 Democratic senators also can be subdivided into four rough groupings based on their scores.
The conservative wing of the Democratic delegation consists of Carlos Uresti of San Antonio, John Whitmire of Houston, and Juan "Chuy" Hinojosa of McAllen. Uresti earns the title of most conservative Democrat.
The line separating the second and third groups is gray. The second group contains the ideological midpoint of the Democratic delegation (between Judith Zaffirini of Laredo and Leticia Van de Putte of San Antonio), and includes those two senators as well as Eddie Lucio Jr. of Brownsville and Royce West of Dallas. All four possess substantial ideological overlap with their Democratic colleagues, only significantly more or less conservative than two fellow Democrats. The third group ranges from Mario Gallegos of Houston to José Rodríguez of El Paso, with Kirk Watson of Austin in between, each noticeably more liberal than over a quarter of their fellow Democrats.
Wendy Davis of Fort Worth and Rodney Ellis of Houston are the Senate's two most liberal members. Each is significantly more liberal than five of their copartisans as well as all 19 Republicans.
Davis' position at the liberal edge of the Senate ideological spectrum is worthy of note, as she is the only Democrat who represented anything even approaching a Republican-leaning district (e.g., the Republican share of the two-party Railroad Commissioner vote in her Senate District 10 was 52 percent in 2008 and 59 percent in 2010). Far from tilting toward the center as many Democrats in swing districts are wont to do, Davis instead embraced her ideological base, with the legislative voting record of an unabashed Texas liberal.
Mark P. Jones is the James A. Baker III Institute for Public Policy's Fellow in Political Science and the chairman of the Department of Political Science at Rice University.