In the UK the phrase "middle class" refers to a large proportion of the population who, broadly speaking, earn more money than they need to pay rent and bills, clothe themselves, eat food etc. There are of course many many subcategories and the terminology itself is slightly outdated now but about 75% of people currently label themselves as middle class.
A British politician could never stand on a platform saying they wanted to support the middle class specifically as this would be tantamount to saying that they didn't care about 25% of the country who are poor and/or working class people and this would be political suicide.
However, in US political speech, I can hardly hear a politician speak without their espousing their love of the middle class. For example the title of this US presidential website is "A better bargain for the middle class".
What does "middle class" mean in this context in the US and why don't Americans also hear this as a callous disregard for the poorest sections of society?
There have been some interesting answers but I feel they are at least partially missing the point. It is also true that in the UK if you could get 75% of people to vote for you you would be very happy. However, in the UK if you explicitly said "We are the party of the middle class", not even many people who were middle class would find this attractive. I wonder if in American "middle class" means hard working or something similar rather than really being a class description?