Liberalism disagrees with realism/political realism on many key assumptions.
There are a number of differences between these two schools of thought.
To begin, unlike political realism, which views the state as the primary actor, liberalism/pluralism sees non-state actors as highly important in the international system. Liberals disagree with realism/political realism about the sole importance of the state. While the state does indeed matter in the international system, individual actors are key in international relations. Related to this, unlike realists, liberals believe that domestic politics should not be ignored.
Thus, they place a primary emphasis on the actions and interests of individuals and groups, and namely these interests within a state. Thus, liberalism argues that “domestic state-society relations constitute the central issue of politics” (Moravcsik, Liberal International Relations Theory: 7). And, unlike realism, which emphasizes individual loyalty to the overall state, liberalism argues otherwise, saying instead that individuals have their own interests, which often can differ from that of government leaders.
As Moravcsik explains, “Private individuals independently calculate personal gains and losses from foreign policy, popular support for foreign policy initiatives, for government institutions and, indeed, for the survival of the state itself, all depend fundamentally on the precise nature of individual preferences and their relation to the international environment”. Thus, liberalism clearly suggests that individuals within a society can have very different calculated goals compared to a state leader/leaders; the idea of a unified domestic front for a state does not exist (or easily can not exist). As Moravcsik explains, “for Liberals, the foreign policy preferences of governments are directly influenced by the formal representative institutions that link state and society. These domestic “transmission belts” include political parties, electoral systems and bureaucracies.
This is a point that counters realist claims about the state being a unified actor. However, this is not to suggest that these individual interests are always harmonious; some within liberalism recognize the conflict associated with the political interests of varying actors.