Moderate presidential hopefuls face a collective action problem—each wants to see voters rally behind one alternative to Bernie Sanders, but (so far) none are willing to quit the race in order to make it happen.And while Joe Biden was once the uncontested front-runner in South Carolina, lavish spending by Tom Steyer and an uptick in attention from the Sanders campaign means that Biden's chances aren't what they once were. This episode: congressional correspondent Susan Davis, senior political editor-correspondents Domenico Montanaro and Ron Elving.