The United Kingdom's full name is the United Kingdom of Great Britain and Northern Ireland. Thus, technically, referring to the UK as "Great Britain" excludes Northern Ireland.
Great Britain is used to formally refer to the UK (including Northern Ireland) in a number of international associations. For example:
The term might not be technically correct, but its use to refer to the UK is certainly not limited to US politicians.
People use Great Britain, at least in my encounters, to talk about England proper (i.e. the part with London). The problem with calling it the United Kingdom is that the UK contains: Scotland, Wales, Northern Ireland, and Great Britain. I tend to find that people typical aren't referring to all of these entities, and they specify which one by using these names. What perplexes me is why the name is the United Kingdom of Great Britain and Northern Ireland. The name leaves out many nations that are in the UK.