Below we use Optimal Classification (OC) in R to plot the Senate’s 68-32 vote on passage of the comprehensive immigration reform bill. Fourteen Republican Senators joined all 54 Democratic and Independent Senators, and a two-dimensional spatial model perfectly captures the divide among Senate Republicans on the vote, producing no classification errors. With the exceptions of Sens. Dean Heller (R-NV) and Marco Rubio (R-FL), these fourteen Republican Yea votes are among the most ideologically moderate members of their party’s caucus. Running OC in one dimension produces only two classification errors (Heller and Rubio are predicted Nays), indicating that this vote almost perfectly conforms to the liberal-conservative divide.
In the second plot below, we use the location of the ideological divide in the Senate vote to predict how the House’s vote on the bill. To do so, we use members’ DW-NOMINATE first dimension (liberal-conservative) Common Space scores, which are directly comparable across both chambers. The mean Common Space score of the 14 Senate Republican Yea votes is 0.403; for the 32 Republican Nay votes it is 0.510. Roughly then, the ideological midpoint between the two groups is 0.457; members of the Congress to the left of this figure should vote Yea and those to the right should vote Nay. We then regress House Republicans’ Common Space scores on their OC first dimension scores based on roll call votes in the current, 113th Congress, and find the OC score that corresponds to the predicted cutpoint in the Common Space metric. This allows for the inclusion of freshman members of the 113th House who do not yet have Common Space scores.
Though there is of course considerable uncertainty in this estimate, we predict that the immigration reform package would pass the House by a large 303-128 margin. But crucially, by this projection, House Republicans would split 103-128 against the bill, and so it would never come to a vote were Speaker Boehner to strictly adhere to the “Hastert Rule.” Again, this is a very tenuous prediction (for example, Rep. Paul Ryan (R-WI) is among the more conservative members of the 113th House [tied for 44th] but is also an advocate for immigration reform). However, it does suggest that House Republicans are fairly evenly divided on immigration reform, at least based on their latent ideological positions. This is also a prediction that we can improve as more House Republicans begin to announce their voting intentions on the bill.