They were a buffer between the working class and the wealthy, usually university educated and earning enough to feed themselves, house themselves, pay for medical care, educate their children beyond public school, take yearly vacations, afford some household help (whether or not they had it) and save for retirement. I grew up in such a household.
Then first the OPEC oil shocks came along and their inflation blamed on working people earning too much money and wages never pretended to keep pace with double digit inflation. Then double digit interest rates hit us. Then Reagan got in and skewed the tax rates, starving government to the point that states had to make up the difference by hiking all the regressive taxes on working people. Everybody but the 1% took a beating.
My dad did surprise me with a portfolio on his death that has given me enough to live on, although in the mid 5 figures I have still not achieved the middle class lifestyle. I think that for 98% of us, that lifestyle might be gone forever.
However, it was the middle class as I defined it that gave stability to the country, that was able to educate the next generation of professionals and leaders without burdening them with debt, and that promised the working class that there just might be enough social mobility out there to make things better for their own children, if those children worked hard to become scholarship material. They also provided seed money for entrepreneurship and the bulk of the revenue to both the Feds and Social Security. All this was destroyed by the war against wages that became a war against everybody but the wealthy.
This was the middle class, how it lived and how it should now be defined. You'll notice I didn't assign a dollar amount because it is different if you live in a coastal city or East Buttcrack in flyover country. It is now largely gone, the lifestyle existing mostly on debt rather than income these days.
Large, strong and stable middle classes are an aberration and don't exist unless governments are there to nurture them and provide them with the tools to grow and flourish. The war on wages that became a tax war has ensured that those tools have been taken away. People who should now be middle class are burdened with debt that puts them into the same paycheck to paycheck lifestyle as labor.
If we want a middle class in this country (and it's in the interest of everyone, even the 1% to have one), we are going to have to find people to run for office who know how to grow one.