In the first half of the 19th century, why did the u. s. congress work to keep a balance between the number of slaveholding states and free states? most congressmen on both sides knew that there would be a civil war if one side became more powerful. most northerners did not want to treat african-americans as full citizens, so congress kept slavery legal in some places. if supporters of slavery or abolition had more representatives in the u. s. congress, they would have their way on the issue. the north could not survive without southern agriculture, so most northern congressmen agreed that slavery was necessary.