Yankel, the frum parties would only be in the opposition for almost 40 years. That was the ruling of the chazon ish and all of the other gedolim.
Rav shach said in the 80s that this psak no longer applies. Under Menachem begin, things were different. He was sympathetic to frum people and was the closest thing Israel has ever had to a religious prime minister.
What Rav shachs understanding of the differences were is beyond my understanding; i have emunas chachamim that he saw a significant change which permitted joining the government.
But at the time he said you can’t be a full minister. Under rav chaim kanievsky, this was changed too, and yaakov litzman became the first charedi minister. Again, we only follow what the gedolim say