43% of eligible voters. This doesn't include people who are not eligible.
There was a really interesting book by Huntington, Crozier, and Watanuki, called "The Crisis of Democracy". They concluded that trying to extend democracy past a few basic issues, into creating "positive rights" for special interests, was creating gridlock. Nobody gets what they want and everyone feels meddled with. This is a crisis because the authors were worried that the gridlock would lead to people endorsing charismatic, authoritarian leaders who effect change extra-democratically. They were right on with this analysis. Unfortunately, their recommendation was to control media channels to narrow peoples' preferences to fit the State...And this is what we have seen happen.
The point is that it's not a problem of government not doing a tractable job. The problem is that they are given an intractable job and this yields poor results.
US democracy isn't intended to define a set of rules for how 300 million people are allowed to live. This is impossible because people are different. It is intended to let people best decide how to protect their own ability to decide how they want to live. It was an interesting experiment, and it appears not to have worked. So, you're left with two major options. 1) Force people to live according to how the most vocal special interests want them to live and deal with the ensuing conflict, or 2) Let people self-segregate into communities which have sovereign decision-making authority. Judging from how well countries with greater subsidiarity (e.g. Switzerland) are doing relative to how countries with greater centralization are doing (e.g. US, China, Russia), the latter seems preferable.