Cooper's milkvetch is a widely distributed but poorly documented legume of the northeastern United States and adjacent Canada. As recently as 1988 this species was believed to be so rare throughout its range that it was proposed to the US Fish and Wildlife Service for potential placement on the federal endangered species list. Only 12 of these populations rangewide were known to be extant, and only two of them in Minnesota. It was downlisted from special concern to non-listed in Minnesota's last revision of the state's endangered species list because surveys subsequent to 1988 have documented an additional 166 populations in the state. Early Minnesota collections suggested that in Minnesota the species was a denizen of lakeshores.
More recent collections indicate otherwise.
The species is now documented from 12 northwestern Minnesota counties. Six populations are known from areas designated as brush prairie and four are on recently disturbed soil. Those populations in prairie generally occur in areas where woody invasion is occurring, or where it is being set back by prescribed fire, leaving a zone with lower cover of prairie grasses and a mix of shrubby and prairie species. Recently-discovered populations in forested counties occur most frequently in openings characterized by higher light levels or more opportunity for soil disturbance than in the surrounding closed forests. The majority of such sites are in early successional microhabitats associated with oak and aspen forests, such as old logging trails, steep banks and public water accesses.