As much as liberals in general will hate to hear this, I think they are mostly incorrect when they state that there is in fact a substantive difference between the Republicans and Democrats — on domestic policy, at least.
I think the one substantive domestic policy difference there is that though the Republicans and Democrats both bow completely and utterly to plutocratic interests, and produce on average about the same end results, the Democrats are less cruel about it. Sometimes in rhetoric only, and sometimes in actual application of socially regressive and oppressive policies, the Democrats don’t actually seem to feel (or at least to express) glee when they subjugate the poor, disentangle the social fabric, and in general make the world worse for everyone.
The Democrats, at least the ones who make it into office, do these things just the same as Republicans, though. Welfare "reform" was signed into law by Bill Clinton, after all. And the Democrats are more firmly in the pockets of the copyright monopolies than the Republicans.
On the foreign affairs front, though, there is more of an argument for the superiority of Democrats (though I rarely if ever hear that argument made by actual liberals). The war in Iraq likely would not have occurred if Bush had never been elected, for instance.
That is, I agree, enough reason to always vote Democrat. However, as I noted above, I rarely ever hear that actual argument. What I hear instead is some absolute bullshit about how Democrats "care" more about the poor, about the middle class, about the working person, when in fact the evidence does not at all bear this out.
They talk a better game, but the result is the same — they just are more clever about it, as often the appearance of Democrat-led false reforms precludes and prevents real reform. This is by design. It’s sometimes referred to as “good cop, bad cop.” And just like in a police force, both cops here are tools of the system, just as are their cheerleaders.
I can understand the appeal of someone not sneering at you while ripping you off and stealing your livelihood, but that doesn’t make the theft any less larcenous, and that doesn’t make the impoverished any less poor.
Let’s not forget that when we talk about the saintliness of Democrats versus Republicans.