The very richest Americans have financed a sophisticated and astonishingly effective apparatus for shielding their fortunes. Some call it the income defense industry," consisting of a high-priced phalanx of lawyers, estate planners, lobbyists and anti-tax activists. All are among a small group providing much of the early cash for the 2016 presidential campaign. Operating largely out of public view - in tax court, through arcane legislative provisions and in private negotiations with the Internal Revenue Service - the wealthy have used their influence to steadily whittle away at the government's ability to tax them. The effect has been to create a kind of private tax system, catering to only several thousand Americans. Two decades ago ... the 400 highest-earning taxpayers in America paid nearly 27 percent of their income in federal taxes, according to I.R.S. data. By 2012 ... that figure had fallen to less than 17 percent, which is just slightly more than the typical family making $100,000 annually. Some of the biggest current tax battles are being waged by some of the most generous supporters of 2016 candidates. Whatever tax rates Congress sets, the actual rates paid by the ultra-wealthy tend to fall over time as they exploit their numerous advantages.