What It Is
I suppose I should explain the basic mechanics of the national popular vote campaign. The animating idea is to take advantage of the fact that states have, under the constitution, essentially unlimited authority to allocate their electoral votes however they want. NPV encourages states to pass laws saying that their electoral votes will be allocated to the popular vote winner if and only if enough states to comprise a majority in the electoral college also adopt such laws. In short, when a state passes an NPV law nothing happens at all in the short term. But if enough states do it to "win" 270 electoral votes, then suddenly there's a gestalt shift and the country has, in effect, a popular vote system.
It's a good idea and it's made a lot of progress in recent years. Chris Pearson reports, however, that somewhat oddly this has become a partisan issue at the state level with Republicans usually in opposition. This he plausibly speculates is the legacy of 2000 where people see NPV as not much more than an effort to degrade the legitimacy of the Bush presidency. In fact, though, this should really just be seen as something that pits state against state. The currently "uncompetitive" states -- places like California and New York and Vermont and Massachusetts but also places like Texas and Alabama and Mississippi and Utah -- are the ones mostly clearly disadvantaged by the current system and that goes for Democrats and Republicans alike.