As a group of homeless day laborers cluster around her in a dusty Lake Worth parking lot, Sister Rachel Sena takes names and numbers. The Catholic nun doles out cash to men and women from such far-flung provinces as Huehuetenango, Guatemala, all recently evicted from overcrowded apartments for code violations. She tells them where they can find shelter and medical care. She does not ask for Green Cards. If some members of Congress have their way, Sena's work would soon be a crime. A proposed federal law could make it illegal for individuals like her to help undocumented immigrants. Sena...