Oil buys US Congress votes, but where does their power come from?

Yes, money buys influence. But why can't any limits be put on their political power? Because millions of people continue to demand their product, that is why. To end their power, stop buying the product.

Oil money behind Keystone XL Senate bill breaks 20 million dollars | Oil Change International: "In other words, those Senators cosponsoring the pro-Keystone XL bill introduced today have received roughly 5 times more in oil industry cash than those who remained off the bill."