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Thirty-one Democrats broke ranks with the rest of their caucus on Friday to vote in favor of approving the Keystone XL pipeline, a project that would shunt a type of oil from Alberta, Canada, to the Gulf Coast.

What's particularly interesting about the votes is how they were distributed. One might expect members of Congress from regions near the construction route (Montana to Nebraska, essentially) to offer their support -- but as you might expect, there aren't a lot of Democrats in that stretch. Instead, it's mostly a mix of Southern Democrats and Democrats in contested districts. (The latter of which shouldn't come as a surprise, given that it's primarily a topic of conversation to try and save embattled Louisiana Democrat Sen. Mary Landrieu.)

There are a few safe Democrats in blue states in the mix, like Rep. Albio Sires (D-N.J.). (Sires has supported the pipeline since the beginning.)

A full list of the representatives is at the bottom of this post, along with their districts' Partisan Voting Index scores.

It's not all politics. One of the rationales for approval offered by Landrieu from the floor of the Senate earlier this week was that shipping oil by rail was more environmentally friendly than shipping it by truck or train, given the frequency with which the latter methods can cause spills from accidents or derailments. Earlier this year, Rep. Sean Patrick Maloney -- one of the New York Democrats who backed Keystone XL -- introduced a bill focused on railway oil shipments after a series of derailments near his district.

On Thursday, the Energy Information Administration noted that rail shipments of oil were up 13 percent in October of this year versus last. That's largely due to the expansion of drilling in the Montana/North Dakota region. Keystone XL would transport oil from that region, too.

Why, then, do so many Democrats oppose the pipeline? Simply put, because of climate change. The project may be critical to making possible extraction of oil from Alberta's oil sands deposits. That means more oil burned, creating more greenhouse gases. (Tar sands oil is also more carbon-intensive while being produced.)

It's an interesting political calculus, but one that largely doesn't affect the future of the pipeline. As it always has, the final decision will rest with the president. How he balances the considerations at play remains to be determined.

Philip Bump writes about politics for The Fix. He previously wrote for The Wire, the news blog of The Atlantic magazine. He has contributed to The Daily Beast, The Atlantic, The Daily, and the Huffington Post. Philip is based in New York City.

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The Fix: The 31 House Democrats that voted for the Keystone XL pipeline, mapped