While DuBois had more formal education and was a leading scholar, Harrison known as the "Black Socrates" or as a "walking encyclopedia," was acknowledged as the foremost Black intellectual of the time. And like DuBois and Garvey, he was an activist—considered the "father of Harlem radicalism." Before Garvey's arrival, Harlem belonged to Harrison. He started the "New Negro" movement, the "Buy Black" campaign, and coined the concept "Race First." He founded the Afro American Liberty League (AALL) and the Voice, as a radical alternative to the NAACP. The Liberty League advocated internationalism, political independence, and class and race consciousness.
W.A. Domingo, who knew Garvey in Jamaica, introduced his to Harrison. Garvey was Harrison's keynote speaker when he launched the AALL. Garvey worked with the Liberty League, before setting up the UNIA in America. The two organizations formed a working relationship and by early 1920, Harrison became principal editor of the Negro World, the official voice of UNIA (Domingo was its first editor). Under Harrison's editorship, the Negro World became the leading race-conscious, radical and literary publication of the day. However, by late 1920, Harrison had grown increasingly critical of Garvey. His attacks were less vitriolic and personal than DuBois', Randolph's or Owen's; moreover, he nevertheless contributed to the UNIA's "Declaration of the Negro Peoples of the World" and continued to write for the paper for two more years.
One of the problems Harrison had with Garvey, was he found the UNIA not class-conscious enough. Harrison was at one time the leading Black socialist in America, and although he left organized socialism, he maintained an anti-capitalist and anti-imperialist stance. He was at odds with what he called Garvey's "desire for empire." In Garveyism, he saw a form of imperialism—Afrikan Diasporan against Afrika. In contrast to Garvey, Harrison emphasized that the Afrikan Americans' principal struggle was in the United States, not in Africa. Afrikans were to free themselves from imperialism by their own hands and effort. Apart from ideology, Harrison also disliked Garvey's iron-fisted style of leadership, his larger than like persona, his emotional appeal to his followers, and the "pomp and circumstances" of the UNIA.
Garvey was a capitalist and suspicion of Communism and Socialism and this put his at odds with all the so-called Black radicals. Garvey believed that Communism would be more beneficial for whites by solving their own political and economic problems but would further limit the success of Blacks rising together. He believed that the communist party wanted to use the Afrikan American vote "to smash and overthrow" the capitalistic white majority to "put their majority group or race still in power, not only as Communists but as white men." For Garvey, it mattered little whether it was communism or socialism, if it was white led, it would only abet white supremacy. Communist were another group of white men who wanted to manipulate Blacks so as to continue to control us. Garvey says "It is a dangerous theory of economic and political reformation because it seeks to put government in the hands of an ignorant white mass who have not been able to destroy their natural prejudices towards Negroes and other non-white people. While it may be a good thing for them, it will be a bad thing for the Negroes who will fall under the government of the most ignorant, prejudiced class of the white race." Harrison, though he differed with Garvey, was the most principled in his disagreement.
After breaking with him, Harrison was the most principled of Garvey's detractors. (Though, Harrison is one of the individuals Garvey's first wife Amy Ashwood supposedly had an adulterous affair with.) Rather than become fixated with destroying the UNIA, Harrison looked to develop political alternatives to Garveyism. He worked with various groups, including the Virgin Island Congressional Council, the Democratic Party, the Farmer-Labor Party, the single tax movement, the American Friends Service Committee, the Urban League, the American Negro Labor Congress, and the Workers (Communist) Party (the name at that time of the Communist Party USA). In 1924, Harrison founded the International Colored Unity League (ICUL), which was his most broadly unitary effort. He would edit the ICUL's Voice of the Negro until his untimely death in 1927.