The Dems respond mostly to pressure from capital too. Their game is to put over their responses on their base, which is different from the Republican one. They are therefore more skilled at talking the reformist talk at primary and election time, and, it's true, a little more hesitant and temporizing in their pursuit of the neo-liberal/ imperialist agenda. But the agenda differs only tactically. You say that the Dems are more susceptible to pressure, but what exactly are the mechanisms for exerting it? The old popular mass organizations (unions, black mass orgs., etc) have been reduced to a handful. There are no big strikes or freedom marches taking place.The main mechanism I can see is withholding votes--the very thing you argue against.
The larger problem with your kind of classical reform v.revolution arguments is that they are out of date--by decades. The choice is not between a party of reform and a party of stasis, but between a party of fast-track capitalist attack, and one of slower-track attack. But both are on the attack, and neither is a means of defense or a "salient starting point" for anything but further defeats.
Jim