An effective attack on Democrats would focus on how hypocritical they are in their claims of being liberal and progressive. With out changing the Libertarian philosophy in the slightest, with a good dictionary, with a slight modification of rhetoric, the Libertarians could present themselves as the only party that that is truly liberal and progressive in America today. Out lefting the left, and attacking the Democrats on their own turf, would more than adequately take care of the problems you envision, while continuing the attack on Republicans.
Well, I guess you and I really have nothing in common, then. I have contempt for Libertarians who try to curry favor with stupid voters by presenting themselves as true liberals. But if you really see it that way, God help you.