We study a setting in which a community wishes to identify a strongly supported proposal from a large space of alternatives, in order to change the status quo. We describe a process -- called deliberation -- in which agents dynamically form coalitions around proposals they prefer over the status quo. We formulate conditions (on the space of proposals and on the ways in which coalitions are formed) that guarantee deliberation to succeed, that is, to terminate by identifying a majority-supported proposal with largest possible support, as long as such a proposal exists. Our results provide theoretical foundations for the analysis of deliberative processes in systems for democratic deliberation support, such as, e.g., LiquidFeedback.