It was made up of the Nationalists and Communists.
The Guomindang (Wade-Giles: Kuomintang), or Chinese Nationalist Party, was China’s largest revolutionary and republican party – at least until the late 1930s.
The Guomindang’s primary mission was to unify China under a republican government. Formed by Sun Yixian and his followers in 1912, the Guomindang was the largest party in both houses of the National Assembly, China’s newly formed legislature.
But when autocratic president Yuan Shikai rendered the assembly powerless and dissolved it, he also declared the Guomindang an illegal organisation. Forced into exile, the Guomindang and its leaders launched a 15-year struggle to reunify China and restore a true republican government.
The Guomindang developed its own military arm, the National Revolutionary Army, which finally achieved reunification in 1927-28.
Led by Jiang Jieshi, the Guomindang was able to form a national government and rule China – or most of it – until the Japanese occupation in the late 1930s. - See more at: AlphaHistory.com.
For further reading, refer to MarshallFoundation.org's Statesman – Chapter 05 – China Passage.