Half the country doesn’t pay income taxes. What do they have to lose?
Income taxes are not the only taxes to which people are subjected.
Picking one form of taxation and suggesting (explicitly, as the Democrats do, or implicitly, as the Republicans do) that folks who don't pay that tax (yet are subject to other taxes) are somehow not paying their "fair share" is nothing short of engaging in rhetorical class warfare.
Of course, all of this discussion masks the real problems: one, our system of government has become excessively complex, convoluted, and bureaucratic that it's nearly impossible to determine exactly whether the government machinery is fulfilling its objectives at a minimal (or near minimal) cost; and, two, the average self-interested American taxpayer, seeking to minimize taxes and fees paid while maximizing benefits obtained from the government ("getting the most bang for the buck," is more than willing to elect officials who add debt - lots of debt - to the mix. In short, the complexity of government has created the illusion that a free lunch - or as my economist friends might put it, both guns and butter at the same time - is in fact possible (while in reality, it's not).