The Fraser Institute released an interesting "Study" today about American Oil imports and specifically imports from Canada. The report ranks and compares the civil, political, and economic freedoms from each of the jurisdictions that America imports Oil (and Gas) from - and Canada comes out pretty much at the top of that list.
Canada supplied the US with over 20% of its Oil imports in 2009, and is the largest supplier of imported Oil to America.
So, when certain interests in Washington try to restrict imports from Canada (as well as bad-mouth the oilsands portion of Canadian production), they are effectively trying to reduce Canadian imports and effectively move that energy supply to repressive governments in the Middle East and other locals.
The report is an informative read. It can be downloaded from the Fraser Institute here.