Below we use Optimal Classification (OC) in R to plot the Senate’s 54-46 vote to reject an amendment by Senators Joe Manchin (D-WV) and Pat Toomey (R-PA) that would require background checks for gun purchases with exemptions for non-commercial firearm sales among family and friends.
Four Democrats (Senators Max Baucus (D-MT), Mark Begich (D-AK), Heidi Heitkamp (D-ND), and Mark Pryor (D-AR)) crossed party lines to vote Nay on the amendment, while Four Republicans (Senators Senators Susan Collins (R-ME), Mark Kirk (R-IL), John McCain (R-AZ), and Pat Toomey (R-PA)) voted Yea. Majority Leader Senator Harry Reid (D-NV) voted Nay, but only as a procedural measure so that he can bring the proposal back to the floor for another vote.
In the second plot we merge this vote with the Senate’s April 11 vote to invoke cloture and end debate on the bill. We see that the cutting lines (that divide spatially predicted Yea votes from spatially predicted Nay votes) from the two votes form three regions in the ideological space: those on the left who voted Yea both times, those (mostly moderate Republicans) who voted Yea on cloture but Nay on the Manchin-Toomey amendment, and those on the right who voted Nay both times.
From a spatial perspective, if the next effort to enact mandatory background checks for gun purchases is to be successful, proponents will likely need to pick up the five votes from among the fifteen Yea/Nay Senators we plot in black.
On the Democratic side, this includes Senators Max Baucus (D-MT) and Heidi Heitkamp (D-ND), though they seem unlikely to flip (see here and here), and Senator Baucus is facing re-election next year. The states represented by several of the thirteen Yea/Nay Republican Senators have also been included in New York City Mayor Michael Bloomberg’s ad blitz on gun control. This is entirely consistent with a spatial analysis of voting patterns on the issue, as most of these Senators (for example, Georgia Republican Senators Saxby Chambliss and Johny Isakson) are within a spatial region that indicates they have the highest propensity to switch in support of the bill. Of course, these Senators are only likely to vote Yea relative to the most conservative contingent of the Senate Republican caucus.
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As a technical note, we are able to plot members of the 113th Senate so early in the new Congress by combining the roll call voting records of returning members in the 113th Senate with the 103 roll call votes held so far in the 113th Senate. Until more votes are compiled for the freshman members, the estimation of their ideal points will remain somewhat imprecise, but by bridging data from the 112th this problem will not be too severe.