This is the question posed yesterday by David Allen Green in the post here. While he suggests some broad guidelines in thinking about the issue, he doesn't pretend to have any cut-and-dried answer. And you can see why it's difficult to come up with one. As David suggests, on the one hand not any old political reason provides an adequate justification for breaking the law; at the same time, in the face of flagrantly unjust laws it could be right to break them (he gives the example of 'helping a member of a persecuted group to escape capture and execution' where that would be a criminal act). So, there is the difficulty. How and where does one draw a line between laws which one finds unpleasant, disagreeable, even unfair to an extent, and those that sit somewhere between gravely unjust and downright monstrous?
I don't have a neat answer to this question either. Each person consults his or her conscience, his or her deepest values. However, there is something more that can be said, I think, of a general, indicative kind. Anyone who lives in a liberal and democratic society and who is a committed democrat accepts, just in virtue of being one, that there will be political decisions, policies, laws, with which they disagree yet go along with nonetheless - opposing them by lawful methods only (argument, campaigning, etc). Not to accept this is to appeal implicitly to a 'rule of dictatorship', according to which 'I get to decide' which of the outcomes of democracy I go along with - which is not being any kind of democrat. That provides at least a rough-and-ready test for answering David's question. Before she can feel justified in breaking it, a law has not merely to be one that the democratic citizen dissents from; it must involve an injustice she judges to be so great that it violates a principle which is, for her, of greater moral weight than her commitment to democracy itself, to abiding by democratic outcomes. To put it another way, it must involve an injustice she judges to be so great that it violates a principle of greater moral weight than that of equality of political rights within the polity, the very principle of democracy itself. This sets a pretty high bar, I would think, for anyone who is indeed of serious democratic outlook.