I'd qualify the middle class as people who are "not poor" and people who "do not have enough wealth in investments so that they still need to work for a living."
So, given that ... I'd look to see which candidates want to lower the taxes on earned income and instead shift that burden over to capital gains.
Or I'd look to see which candidates want to go for government health care, as medical costs can turn somebody into "poor" from "middle class" overnight.
I'd also look for a candidate who has less than 1,000,000 in their "war chest", and who does not take any donations from large corporate masters.
So, my answer is that there is no candidate that even claims to serve the middle class.