The Republican party has not been "grassroots" for a while. They have been very "top down" for quite a while now. There are some powerhouse players in that party that like to stay behind the scenes. They had a great strategy that worked for a while: put on fake grass-roots campaigns and rally the troops to your agenda. The party talks to the talk show hosts, and the talk show hosts spread the gospel. That worked until the Immigration debate hit. The immigration issue is a true Republican grass-roots phenomena, and it has thrown the party for a loop. This points to a disconnect between the leadership and the masses.
Meanwhile, the Democratic party has always been decentralized. The Democratic party is best seen as a coaltion of groups, each one moving itself along. Of particular note is MoveOn.org, which built itself from nothing into a Democratic powerhouse. Obama worked with the grass roots organization, and the results have been spectacular. They used their considerable skills in coalition building and grass-roots organizing to change the political landscape. It is a powerful machine. The trick about this machine is that it is based on trust. You must trust that your coalition will pull for you. How do you do that? You must always speak to their vision, and encourage them to action. When we look at Obama's style, that is the leadership that we see. It is no surprise that a man whose foundation is in community organizing and coalition building uses these tools to build and lead his campaign.
Right now, the Republlicans can't match this. Their ideal of a solid, united block is founded on pulling together into your own group. That works great when your group is defensive. However, when your group is feelling disaffected, and your administration has left your group feeling defensive, maintaining that block becomes difficult.
Even worse, when you start needing allies in your own coaltion, you have no apparatus other than fear, uncertainty, and doubt to gather them. That does not build you a coalition, nor does it build you a majority.
I think where the Republiclan party went wrong is when they conceived a permanent majority in the government. Once they got that idea, they organized around forming and maintaining a block. In essence, once they were the only game, you would be forced to play their game. That is the wrong dynamic. In truth, the Republican party, like the Democratic party, is a coaltion. You must always be reaching out to your potential allies, showing that you have room for them. You must also be reaching to your own coaltion, showing that they may be disaffected, but their voices matter. When you form a block of "with us or against us", you will shed that part of your own party that feels ambivalent. Combine that with gaining no new allies, and you can see a recipe for ineffectivness.