It's The Heart, Not The Spine
I think the spinelessness analysis gives the Democrats too much credit. It's based on the presumption that they want progressive change, but are simply too scared to fight for it.
In reality, I think the Democrats are basically moderate conservatives at heart. They love big business, and lack sympathy for the situation of women, people of color, LGBT people, and other disadvantaged groups (even, in many cases, if they happen to be members of those groups). While each party may have attracted a few true believers, the typical Democrat is not that much different in their real views from the typical Republican.
American politics is a contest between two elite factions who both have the same policy preferences. The catch is that to gain the power they want, they have to win elections under a quasi-democratic system, which means appealing to some base of support in the wider population. The Republicans, for whatever historical reason, have claimed the more straightforward constituency -- big business and cultural conservatives, that is, the people who want what both elite factions want to give them. The Democrats, on the other hand, have looked to a tricker base of support -- the portion of the electorate that wants something different from the conservative program. This is potentially a larger base of support, but it's a tough one to depend on, because they have no interest in giving these voters what they want. So the Democrat Party carefully positions itself just slightly to the left of the Republicans. That way, whenever their base starts to get restless, they can point out how the GOP is worse.
When the Democrats drop the ball on some issue, it's not because they're too spineless to carry it. It's because they don't really like that ball, and they were looking for any excuse to be able to ditch it.