Complex sentence: In that occasion the PDSI, that was especially strong in Southern Italy, gained 10.9% of the vote and 60 seats in the Chamber of Deputies, while in 1921 it won 4.7% of the vote and 29 seats.
Split into simpler sentences: In that occasion the PDSI, that was especially strong in Southern Italy, gained 10.9% of the vote and 60 seats in the Chamber of Deputies. Four years later, in 1921 general election the PDSI won only 4.7% of the vote and 29 seats.