I integrate qualitative and quantitative evidence to shed light on the reversal in resource mobilization effectiveness between the CCP and the KMT. I show that the Sino-Japanese War fundamentally shifts the comparative advantages of party mobilization infrastructures of these two parties. The Japanese occupation significantly weakened the KMT by undermining the economic elites upon whom its elite mobilization infrastructure relied. Meanwhile, the CCP was able to take advantage of its mass mobilization infrastructure in rural areas to respond to the wartime fiscal shocks. At the point when both parties were forced to extract grains as an alternative source of revenue after 1941 to address rising fiscal demand, my research uncovers a surprising pattern: the CCP developed a significantly stronger capacity for grain mobilization than the KMT. I demonstrate that the CCP employed its grassroots party organization to mobilize compliance in the peasantry, maintaining popular support even in the face of a significantly higher degree of extraction. The KMT, by contrast, relied on local elites for grain extraction, which generated regressive taxation and corruption, stirring mass resentment despite a lower grain burden.