Maritain is the philosopher who moved Catholic political thought from throne-and-altar reaction to the defense of liberal democracy, and did it from inside the most traditional resources available.
A convert who came to Aquinas through Bergson, he spent the 1920s associated with the reactionary right and broke with it decisively when Rome condemned Action Française in 1926. What followed was an argument, worked out through the 1930s and 40s, that the Thomist tradition properly understood requires democracy rather than merely tolerating it.
The pivot is his distinction between the individual and the person. As individuals we are parts of society, subordinate to the common good; as persons — beings with a spiritual destiny that transcends the political order — we possess a dignity no state may override, and the state exists to serve it. This gives him a foundation for inviolable rights that is neither liberal individualism nor collectivism, and a doctrine he called integral humanism: a Christian politics that is pluralist and secular in form, seeking a society inspired by Christian principles rather than one governed by the Church.
He was in America when France fell, stayed, and became a leading voice against Vichy. After the war, as ambassador to the Holy See and at UNESCO, he took a central part in the drafting of the Universal Declaration of Human Rights, and made the argument that made agreement possible: that people who disagree fundamentally about why humans have rights can nevertheless converge on which rights they have — "we agree on these rights, providing we are not asked why".
His influence runs directly into Christian Democracy, Vatican II, and the Church's eventual accommodation with religious liberty.