In the 1949 elections the party gained five seats, with Idov Cohen, Yeshayahu Forder, Avraham Granot, Yizhar Harari and Pinchas Rosen taking their place as Members of the Knesset (MKs). They joined the government as a coalition partner of David Ben-Gurion's Mapai party, and were members of both the first and second governments.
In the 1951 elections the party lost a seat and dropped to 4 MKs. They were not included in Ben-Gurion's original coalition, but were brought into the fourth government as a replacement for the religious parties Agudat Israel and Agudat Israel Workers who had resigned over religious education issues. They were also a coalition partner in the fifth government (created when Ben Gurion resigned and was replaced by Moshe Sharett), but were dropped from the sixth government after a motion of no-confidence had been brought against the ruling coalition.
At the 1959 elections the party gained another seat, their representation rising to six MKs. Again they joined Ben-Gurion's coalition. During the fourth Knesset the party merged with the General Zionists to form the Liberal Party. It was the motion of no-confidence brought by the new Liberal Party and Herut that brought down the government.
The 1961 election saw the Liberal Party become the third largest in the Knesset, though they did not join the coalition. Later in the session, the majority of the Liberal Party MKs merged with Herut to form Gahal (which eventually became Likud). However, the MKs that agreed with the merger were largely previous members of the General Zionists. Most former Progressive Party MKs objected to the alliance with Herut and set up the Independent Liberals instead.