The North American Free Trade Agreement is 22 years old — but Donald Trump and Bernie Sanders have put it back in the headlines. They attack it for putting Americans out of work by sending jobs to Mexico. Bill Clinton signed NAFTA, making it awkward for Hillary, especially in California. Economists say free trade is what they call "a net benefit" to the US economy, but until now, there's been more emphasis on the gain than there's been on the pain. With the Trans Pacific Partnership not yet submitted to Congress, we get a reality check on an issue that won't go away.