why did congressional republicans pass amendments to the constitution
To abolish slavery and indentured labor.
Thirteenth Amendment to the United States Constitution
With no Southern states represented, few members of Congress pushed moral and religious arguments in favor of slavery. Democrats who opposed the amendment generally made arguments based on federalism and states' rights. Some argued that the proposed change so violated the spirit of the Constitution it would not be a valid "amendment" but would instead constitute "revolution". Representative White, among other opponents, warned that the amendment would lead to full citizenship for blacks.
wiki:19623418
Thirteenth Amendment to the United States Constitution [SEP] Passage by Congress
The Republican Party platform had, as yet, failed to include a similar plank, though Lincoln endorsed the amendment in a letter accepting his nomination. Frémont withdrew from the race on September 22, 1864 and endorsed Lincoln. With no Southern states represented, few members of Congress pushed moral and religious arguments in favor of slavery. Democrats who opposed the amendment generally made arguments based on federalism and states' rights. Some argued that the proposed change so violated the spirit of the Constitution it would not be a valid "amendment" but would instead constitute "revolution". Representative White, among other opponents, warned that the amendment would lead to full citizenship for blacks.