There are several statistical methods that seek to rate candidates’ ideology on a left-right scale. FiveThirtyEight uses three of these methods in evaluating the ideology of Senate candidates as part of our technique for forecasting those races. The same methods can be applied to presidential candidates.
The first of these systems, DW-Nominate, is based upon a candidate’s voting record in the Congress. The second method, developed by Adam Bonica, a Stanford University political scientist, makes inferences about a candidate’s ideology based on the groups and individuals who have contributed to his campaign. The third method, from the Web site OnTheIssues.org, works by indexing public statements made by the candidate on a variety of major policy issues.
Not every rating system is available for every candidate: those who have never served in Congress have no DW-Nominate score, for example. And the methods sometimes disagree. The libertarian-leaning Senator Rand Paul of Kentucky is rated as being extremely conservative by DW-Nominate and by Mr. Bonica’s method, which tend to give more emphasis to a candidate’s record on economic issues. But he is rated as fairly moderate by OnTheIssues.org, which also evaluates his stances on social policy. Sarah Palin is also rated as extremely conservative by Mr. Bonica’s system, but as relatively moderate by OnTheIssues.org. (Keep in mind that before being selected as John McCain’s running mate, Ms. Palin had some history as a reform-minded governor of Alaska.)
Nevertheless, we can usually get a reasonably good objective measurement of a candidate’s ideology by essentially taking an average of the three approaches. (Because the measures are not on the same scale, I normalize Mr. Bonica’s scores and the OnTheIssues.org scores to give them the same mean and standard deviation as DW-Nominate.) The higher the score, the more conservative the candidate.
DW-Nominate scores normally run on a scale that goes from negative 1 for an extremely liberal candidate to positive 1 for an extremely conservative one. To make the result more legible, I have multiplied all scores by 100 — so that, for instance, a moderate Republican might have a score of 25 rather than 0.25. Mr. Rubio achieves a score of 51 by this method.