The founding ecumenical insight is that there already is some kind of unity among the churches. Every church professes that the true church is one; the difference of opinion is where the boundaries lie around this one church. Time spent together in conversation, prayer, and study has shown that there is a lot more common ground between churches than was previously thought. We do after all confess faith in the same Father, Son, and Holy Spirit, whom we know through the same holy Scriptures.
The question is: is that enough? Does such “spiritual unity” suffice for the unity of the church, or does there need to be some kind of organizational unity too?
The answer to that question, of course, varies quite widely. The more congregationalist you are, the more you think organizational unity is irrelevant; the more episcopal you are, the more you think organizational unity is essential. In the space in between these two extremes, generally among churches of presbyterian or synodical organization, the proposed middle ground in the early twentieth century was called “federalism,” much like in secular governments.
This form of unity would allow individual church bodies to maintain their independence but cooperate in very specific ventures, especially in areas of diaconal service or government lobbying. It would give a “visible” profile to the unity without getting bogged down in the details.
The problem is that this model doesn’t really represent “unity” in any meaningful sense. It’s more like a tactical alliance. As the ecumenist John Kent put it: “Christ is more than the president of a federal republic of Christian associations; he is the Head of the Body which is his church.” It’s also hard to locate any spiritual component in it, and the moment one church disagrees with the others it can withdraw. Such unity would be fragile at best.