For millions of voters, Donald Trump and Hillary Clinton present an agonizing choice between the lesser of two evils. Libertarian Gary Johnson offers an alternative for fiscal conservatives who won't vote for Trump but can't stand Hillary. The Green Party's Jill Stein speaks to disappointed and angry supporters of Bernie Sanders. But third-party candidates always pose a moral quandary: can they be anything but spoilers? Do they take votes away from the least of the perceived "evils" and help to elect the worst? We look at this year's competition for the Republicans and the Democrats.