The likelihood of 'mass migration' in the US as a consequence of global warming is about the same as the historical 'mass migration' from the 'low countries' in Europe who have built massive structures to continue living below sea level.

http://www.nytimes.com/2005/09/06/science/in-europe-hightech-flood-control-with-natures-help.html

"So at a cost of some $8 billion over a quarter century, the nation erected a futuristic system of coastal defenses that is admired around the world today as one of the best barriers against the sea's fury -- one that could withstand the kind of storm that happens only once in 10,000 years.
The Dutch case is one of many in which low-lying cities and countries with long histories of flooding have turned science, technology and raw determination into ways of forestalling disaster.
London has built floodgates on the Thames River. Venice is doing it  on the Adriatic."

We rebuilt levees in New Orleans, remember?

 Florida is  a Presidential 'swing State'!