Maybe for Mr. Summers, it is. And maybe as well, for liberals generally.
But there are at least a couple problems. First, many Americans are apparently convinced that some other issue is central in American politics. Maybe it's threats from foreigners. Or from Satan, or some proxy of his. Or perhaps, it's losing their guns. Or whatever.
Second, even among those who agree that the economic security of the middle class is the central issue in American politics, many have been convinced that rewarding wealthy capitalists is the best strategy.
So it goes.
Edit: Indeed, many middle-class Americans think that they're wealthy, and so don't care so much about the middle class.