Namely: it will consume too much energy to ship water from the Great Lakes to the Far West, so why should we worry about conserving water?
It's understandable that potato growers, who use large amounts of water, are worried about state standards for their water use, but this strawperson is too easy to knock down.
A pipeline over the Rockies is not the 'alleged external threat.' It's the possibility of diverting water relatively short distances to the Mississippi River and thence to the Great Plains. It's also the multiple assault of climate change (which could lower the level of Lakes Huron/Michigan five feet this century), private capture and sale of water in tankers and bottles, and population growth and wasteful water use just outside the Great Lakes Basin.
http://www.lsj.com/apps/pbcs.dll/article?AID=/20060105/OPINION02/601050329/1087/opinion