The Keystone pipeline was going to move tar sand bitumen, a very dirt crude product that requires more extensive refining than common crude product. Once refined at a higher cost and with a greater potential for a negative environmental impact it was going to be exported out of the U.S., not added to the domestic supply. IOW, the Keystone Pipeline would add nothing of value to the U.S. market.
I would support the Keystone Pipeline if the tar sand was refined in Canada and only refined product flowed through it.
The Keystone Pipeline system has been moving oil from Alberta to U.S. refineries more-or-less continuously since 2010.
The Keystone XL was a much smaller "shortcut" pipeline to be added to the already existing Keystone system, which would pass through eastern Montana and Nebraska. In eastern Montana the "dirty" Alberta oil would be blended with "sweeter" oil from the Bakken oilfield, making it much easier to refine into cleaner fuel.
There was a BUNCH of resistance to the Keystone XL, due to it passing through the Nebraska Sandhills, which are above the Niobrara Aquifer, which produces a LOT of grain for the U.S. If the XL pipeline leaked it would cause major problems.
But the XL would be only a minor part of the Keystone pipeline system--which wouldn't affect the world price of oil more than (maybe) a few cents.