Throughout 2015, we kept hearing warnings that wealthy individuals would dominate the presidential election, and perhaps even influence the outcome, all because of court-made changes in campaign-finance law that allowed unlimited contributions and spending. So why did Big Money in politics end up to be a big bust in 2015? Super PACs -- political action committees that can accept unlimited amounts from just about anyone but can't coordinate with the candidate -- have been spending money, yet have little to show for it. Other dark-money sources -- essentially nonprofit social-welfare groups that don't have to disclose their donors -- had...