As the battle over the Keystone XL pipeline has worn on — and it’s now well over two years old — it’s illuminated the Obama presidency like no other issue. It offers the president not just a choice of policies, but a choice of friends, worldviews, styles. It’s become an X-ray for a flagging presidency. The stakes are sky-high, and not just for Obama. I’m writing these words from Pittsburgh, amid 7,000 enthusiastic and committed young people gathering to fight global warming, and my guess is that his choice will do much to determine how they see politics in this country.