“An anti-Zionist would not have called for people to vote in Israeli elections, much less found a political party.”
He explains why he did so in one of his sefarim or igros. It was very reluctantly because he was an anti-Zionist and it was only because he felt it was necessary since practically speaking the Zionists were running the country but he was very unhappy about the fact that the zionists were running the country and that this was necessary.
Maybe one of the posters who is better at remembering and locating sources than I am could find the source and post it. I would imagine it’s quoted in “b’mechitzasam” if anyone has that handy.