民主主义 - 'democracy as an -ism' - is the suffix-heavy form preferred in 20th-century political theory writing. Mao Zedong's 1940 essay
《新民主主义论》 ('On New Democracy') used
新民主主义 to label the bourgeois-democratic stage of the Chinese revolution. In casual modern speech,
民主 alone usually suffices for 'democracy';
民主主义 sounds textbook-formal.