There are two Catholic organizations in the United States which work very closely with each other to further the ecumenical mandate of Christ as expressed in John 17:21ff and embraced by the Second Vatican Council in its document, Unitatis Redintegratio, or the Decree on Ecumenism and later in the Directory for the Application of Principles and Norms of Ecumenism (1995). 

The United States Conference of Catholic Bishops, (USCCB) through its Committee on Ecumenical and Interreligious Affairs maintains very active dialogues with almost all Christian denominations or ecclesial communities in the United States.  The Committee has a secretariat in Washington, DC, that staffs the work of the CEIA, as it is known.  

Additionally, every diocese in the country is mandated to have an Ecumenical Officer to facilitate ecumenical relations on the level of the local church.  All of these Ecumenical Officers belong to a network known as the Catholic Association of Diocesan Ecumenical and Interreligious Officers (CADEIO).   As you can imagine the interplay between the USCCB and CADEIO is quite fruitful.  Developments on the national level set a valuable context for the local scene while the local scene helps to inform and enliven events on the national level.