Economists have long pleaded for an overhaul of the corporate tax code, and both President Obama and Republicans now say they favor one, too. But it won’t be easy. Companies that use loopholes to avoid taxes don’t mind the current system, of course, and they have more than a few lobbyists at their disposal. Over the last five years, Boeing paid a total tax rate of just 4.5 percent, according to Capital IQ. Southwest Airlines paid 6.3 percent. And the list goes on: Yahoo paid 7 percent; Prudential Financial, 7.6 percent; General Electric, 14.3 percent.