The environmental wackos have been DOGGING his ass since the reelection - he promised them he would kill it and he hasn't. They are pissed off and he is going to have to veto this to appease them, even tho the vast majority of Americans and most of Congress are in favor of it. Mary Landrieu lost her Louisiana Senate seat because she couldn't get the bill pushed through Congress - that's a good thing, but I digress.
He has nothing to lose, but the Dims in certain states impacted by the pipeline do.
Fuck the greenies, BTW. Once they are built, the pipelines actually can be beneficial to their immediate environment. In all this "climate change" animals actually hang out near the pipelines because the oil flowing through is warm. It's like a big warm spot for the animals to hang out. Fertility and birth rates for some endangered species the greenies claimed would be harmed by the pipelines are actually increasing because the animals get all warm and... frisky...
The jobs created and the long term impact on our ability to tell foreign oil to piss off makes this a big win win situation. But this asshole in the White House has fucktards to appease, so he will veto. And hopefully more Dims will pay the price in November like Landrieu did.