There are 50,000 coal fired energy plants worldwide and 8,000 of them are in the U.S. These plants emit a combined 10 billion mega tonnes of GHG per year into our atmosphere. The U.S. emits 2.8 billion of that. Alberta oil sands development emit 0.045 billion/yr. KXL is an easy target because it's so politicized. If you were really concerened about "exacerbating" GHG emissions you could get a few of your coal plants to reduce emissions. In fact if you just shut down 2 of the top emitters of the approximate 600 plants in the U.S. it would offset the entire annual emissions of the Alberta oil sands. Or maybe just shut down your own "dirty oil" developments in California.
What politicians do not seem to be emphasizing is the fact that the keystone pipeline is an invasion of several indigenous nations' sovereign lands, including the Sac and Fox nation, the Oglalla Lakota, Caddo nation, Rosebud Sioux. All the delegates from the Great Sioux Nation have condemned this "bill" as another illegal rejection of treaties by the US. The construction of this pipeline could have severe environmental impact and heal impacts (for many, they are similar things) for these tribes.