I think the Republicans also have a bit easier of a time getting their voters to fall in line because they've got a basket of wedge issues that a vast majority of their base A) agrees on and B) at least somewhat cares about. Largely due to Christian values and/or social conservatism.
For instance, I think the portion of the Democratic electorate that supports gay marriage, and particularly those that support it enough to base (in whole or part) a voting decision on it, is smaller than the portion of the Republican electorate that feels conversely. Same for abortion, same for faith-based initiatives, same for abstinence-only education, and so on, and so forth.
I don't think the Democrats are nearly as united in social liberalism as the Republicans are in social conservatism. Which means that often when a candidate comes out on <insert wedge issue>, not only is he unlikely to win any Republican voters over, but there's a good chance he'll be driving away some of his own Democratic voters...regardless of his position on the issue.