Below we use Optimal Classification (OC) in R to plot the House’s 252-161 vote and the Senate’s 59-41 vote on approving the Keystone XL pipeline. The Senate fell one vote short of approving the measure, which was designed in part to help Senator Mary Landrieu’s (D-LA) prospects in her upcoming runoff against Rep. Bill Cassidy (R-LA).

31 House Democrats and 14 Senate Democrats joined with all voting Republicans in both chambers in support of the measure. The first dimension (representing legislators’ liberal-conservative positions) does a good job at capturing this internal divide among House and (especially) Senate Democrats. Three of the Democratic Senators who voted Yea were defeated in their re-election races earlier this month — Senators Mark Begich (D-AK), Kay Hagan (D-NC), and Mark Pryor (D-AR) — and these are among the more moderate members of the Senate Democratic Caucus. This helps to illustrate why polarization is likely to increase in the next (114th) Senate.

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