This law was declared unconstitutional by the Supreme Court on the grounds that general taxes were being collected to pay one special group of people.
本句的主干是This law was declared unconstitutional...。on the grounds引导原因状语从句。
unconstitutional为constitutional的反义词，意为“违反宪法的；非立宪的”。the Supreme Court指“最高法院”。on the grounds that意为“因为；以……为理由”。
from the ground up意为“从头开始”。如：
Don't give up. You can do it from the ground up.
The new country will break ground on all aspects.
The leader was losing ground as other runners accelerated.