A historical and descriptive definition: 19th century
The Universalist approach to associationism in the first half of the 19th century had strengths and weaknesses. The decentralization and methodological diversity allowed Universalism to adapt readily to local circumstances, and small Universalist congregations sprang up all over the United States and its territories, and to a lesser extent up into Canada. That same decentralization also meant that there often was no ongoing support and nurture for small new congregations, many of which died out two or three decades after they began.
Meanwhile, the Unitarians found themselves forced into associationism, kicking and screaming as it were. Disestablishment meant that the strongest and most powerful Unitarian congregations suddenly had to learn how to provide their own financial support; not only that, but they also found themselves competing for potential members with a wide range of other denominations (including, of course, the Universalists). The first feeble step towards real associationism came with the establishment of the American Unitarian Association (AUA), so at least there was some central body to distribute Unitarian propaganda; but the AUA was an association of individuals and a few congregations, so it cannot be considered true associationism, an association of congregations, using my definition. The Autumnal Conventions represents the first real emergence of associationism in the Unitarian camp: a few far-sighted individuals decided that delegates from Unitarian congregations needed to meet annually to organize themselves around topics of mutual interest. The Autumnal Conventions were weak associationism, however: many Unitarian congregations did not send delegates (or much care about the Autumnal Conventions), and the Conventions didn’t do all that much.
Associationism among the Unitarians really begins with Henry Whitney Bellows and the National Unitarian Conference in the 1860s; that well-documented story need not be reviewed here. Unitarian associationism is also represented in the old Western Unitarian Conference, which actively promoted connections between congregations, and actively worked to spread Unitarianism in new areas, using innovative methodologies such as encouraging women ministers (e.g., the women ministers known as the “Iowa Sisterhood”). What is important is that Unitarian associationism required neither covenant nor Puritan-style connections between congregations.
On the Universalist side, associationism evolved over the course of the latter half of the 19th century: it had to evolve, because Universalism began to shrink. The methodological and theological diversity, and loose organizational structure, that proved so effective for Universalists in the early 19th century turned out to be a liability in the long run. Early Universalist associationism was too loose, and as local congregations died out for lack of support, Universalists tried to become more tightly organized. They experimented with a creed-like statement in the late 19th century to create theological consistency; conventions tried to exercise more organizational control over local congregations. But the Universalists were not able to focus their efforts effectively.
Many writers have argued that the Universalists began to decline because so many other denominations effectively signed on to the general notion of universal salvation that the Universalists were no longer distinctive. That may be true, but it also seems self-evident that the associationism of the Universalists was not able to evolve quickly enough to keep pace with the societal changes going on. By 1945, there were no Universalist congregations left in Boston, even though the denominational headquarters was in that city; but there were plenty of small Universalist congregations in rural areas, areas which were beginning to see a decline in population. To me, it appears that Universalism declined in the late 19th and early 20th centuries because while Universalist associations were skilled at creating loose associations that functioned well frontier and rural settings, they were unable to organize effectively for the demands of urban environments; yet urbanization, and then suburbanization, were the dominant trends in the United States in the middle 20th century.
A historical and descriptive definition: early 20th century
The Unitarians managed to adapt their associationism to the new demands of urban, and then suburban life. In the late 19th and early 20th centuries, the American Unitarian Association managed to centralize a good deal of power in its executive office, and that centralized power enabled them to fund and organize new congregations, particularly in the Far West. The AUA funded church buildings, and paid the salaries of ministers, trusting that some of the new congregations would become self-supporting. Another turning point in the history of Unitarian associationism happened in the middle of the Great Depression. Small congregations were dying throughout America, unable to cope with the economic situation. A commission that included James Luther Adams studied the situation, and issued a series of recommendations under the title “Unitarians Face a New Age.” Many of these recommendations led to profound changes in Unitarian associationism; for example, rather than trust independent organizations like the Unitarian Sunday School Society to produce religious education materials, the AUA centralized curriculum production by hiring Sophia Fahs as a curriculum editor; her New Beacon Series of curriculums revolutionized both Unitarian theology and life in local congregations, and arguably was one of the two biggest drivers of the phenomenal growth in Unitarianism in the 1950s.
The Universalists managed to pull themselves back from along slow slide into irrelevance by merging with the Unitarians. Merger negotiations continued through the 1950s. Meanwhile, the Unitarians rolled out another innovation in their associationism, the fellowship movement. Unitarians no longer required new congregations to from “churches” with enough people to financially support a paid minister. Suddenly, new congregations could form as “fellowships,” without a paid minister; all a fellowship needed to enter into formal association with the Unitarians was ten people and regular corporate meetings, along with a formal business structure (i.e., bylaws and financial records). With this innovative new methodology, the Unitarians spread themselves into the new frontier of the rapidly growing suburbs.