They were an anti-Chiang faction that split during the late civil war and declared support for the CPC. They still exist on the mainland until this day.
There was also the left-right split before the war with Japan; after Sun Yatsen’s death there were rival KMT governments with the rightists led by Chiang in Nanjing and a leftist government in Wuhan led by Wang Jingwei, though the latter folded and the party reunited until Wang became a traitor during the war and sided with Japan.
The KMT did most of the heavy lifting because they governed almost the entire country while the CPC controlled a single, very poor province.