Here are the ideology and leadership indices for the House and Senate for 2011-2016:
As you can see, the ideology score seems to do a good job of separating the two major parties. Looking at the 2014 scores specifically, it's even clearer. For example Senators Jim Inhofe, John Cornyn, and Pat Roberts all score near 1 on the ideology index (far right on the chart above); Senators Elizabeth Warren, Barbara Boxer, and Bernie Sanders score near 0 (far left on the chart); and Senators Joe Manchin, Lisa Murkowski, and Mark Pryor all score near the middle.
I took the data for the 2013-2014 legislative period and split it into 5 groups: far-left wing, left wing, center, right wing, and far-right wing which allows for non-linearity in the effect of ideology on the probability a legislator voted for the 2014 Farm Bill. I plan to use a few different versions of this breakdown in the paper.
In the model I controlled for the legislators' party, sex, whether they were Senators or Representatives, and whether or not they served on the Ag Committees in their respective houses.
As you can see below, 68% of Senators and about 60% of Representatives voted in favor of the bill (i.e. voted "yea") and only 8 of the 65 members of both Ag Committees voted against it.
Looking at the correlation between a "yea" vote and the raw ideology index from GovTrack shows a weak but statistically significant positive correlation. The interpretation of this is that more right wing legislators were more likely to vote in favor of the bill.
|(click the image to enlarge)|
So estimating a probit model with controls for party, house, sex, and Ag Committee membership gives us the following marginal effects:
Senators and members of the Ag Committee were, as expected, more likely to vote in favor of the 2014 Farm Bill. Compared to far-right wing legislators (the reference group), far-left wing legislators were 29% less likely to vote "yea" on the bill and right-wing legislators were 19.6% more likely to vote for the bill. There was no statistically significant difference in the probability of voting "yea" for left wing and centrist legislators relative to far-right wing legislators.
Looking again at the cross-tabs above, I think the regression analysis shows that there was, at least in the case of the most recent Farm Bill, a coalition of primarily right-wing and centrist legislators banding together with some far-right wing and left-wing legislators to pass the bill. While I have other variables to add to the model, I suspect that these relationships will hold up pretty well given the importance of ideology in other studies of legislative voting behavior. As always, I'm interested in readers' thoughts, suggestions, and questions.
*11:45 PM 4-24-2016 - I had previously stated that SNAP spending increased. Thanks to Keith Coble for the correction.