I think the reasons are not limited to the last 10 years, the American middle class is dead it's been going on for decades. If you stretch the economic data over a longer time frame you can see the decline in a lot of metrics. I would argue the downfall started due to the presidency of Ronald Reagan, while republicans are heavily responsible democrats have not done much to reverse the damage republicans have done. For example Obama kept the Bush tax cuts, Clinton continued the path of deregulation leading to the 2008 financial crisis.
Democrats have been leaning more and more right for decades and at this point the American public is simply lashing out and blaming the usual suspects (immigrants and minorities). Since there is no left wing in this country to give them an alternative and doing something about it, it has become the truth. You can look at it world wide the left wing of the world is too busy protecting corporate interests to make the changes necessary to stop fascism.