As the country had several powerful left-wing groups and it remained fairly small in size. By 1936 it had a membership of only 40,000 people.
On 15th January 1936, Manuel Azaña helped to establish a coalition of parties on the political left to fight the national elections due to take place the following month. This included the Communist Party, the Socialist Party (PSOE), and the Republican Union Party.
The Popular Front, as the coalition became known, advocated the restoration of Catalan autonomy, amnesty for political prisoners, agrarian reform, an end to political blacklists and the payment of damages for property owners who suffered during the revolt of 1934. The Anarchists refused to support the coalition and instead urged people not to vote.
Right-wing groups in Spain formed the National Front. This included the CEDA and the Carlists. The Falange Española did not officially join but most of its members supported the aims of the National Front.
The Spanish people voted on Sunday, 16th February, 1936. Out of a possible 13.5 million voters, over 9,870,000 participated in the 1936 General Election. 4,654,116 people (34.3) voted for the Popular Front, whereas the National Front obtained 4,503,505 (33.2) and the centre parties got 526,615 (5.4). The Popular Front, with 263 seats out of the 473 in the Cortes formed the new government.
The Popular Front government immediately upset the conservatives by releasing all left-wing political prisoners. The government also introduced agrarian reforms that penalized the landed aristocracy. Other measures included transferring right-wing military leaders such as Francisco Franco to posts outside Spain, outlawing the Falange Española and granting Catalonia political and administrative autonomy.
As a result of these measures the wealthy took vast sums of capital out of the country. This created an economic crisis and the value of the peseta declined which damaged trade and tourism. With prices rising workers demanded higher wages. This led to a series of strikes in Spain.
On the 10th May 1936 the conservative Niceto Alcala Zamora was ousted as president and replaced by the left-wing Manuel Azaña. Soon afterwards Spanish Army officers, including Emilio Mola, Francisco Franco, Gonzalo Queipo de Llano and José Sanjurjo, began plotting to overthrow the Popular Front government. This resulted in the outbreak of the Spanish Civil War on 17th July, 1936.
President Manuel Azaña appointed Diego Martinez Barrio as prime minister on 18th July 1936 and asked him to negotiate with the rebels. He contacted Emilio Mola and offered him the post of Minister of War in his government. He refused and when Azaña realized that the Nationalists were unwilling to compromise, he sacked Martinez Barrio and replaced him with José Giral. To protect the Popular Front government, Giral gave orders for arms to be distributed to left-wing organizations that opposed the military uprising.
In September 1936, President Azaña appointed the left-wing socialist, Francisco Largo Caballero as prime minister. Largo Caballero also took over the important role of war minister. Largo Caballero brought into his government two members of the Communist Party: Jesus Hernández (Education) and Vicente Uribe (Agriculture).
The May Riots in 1937 severely damaged the Popular Front government. Communist members of the Cabinet were highly critical of the way Francisco Largo Caballero handled the disturbances. President Manuel Azaña agreed and on 17th May he asked Juan Negrin to form a new government. Negrin was a communist sympathizer and from this date Joseph Stalin obtained more control over the policies of the Republican government
Negrin's government now attempted to bring the Anarchist Brigades under the control of the Republican Army. At first the Anarcho-Syndicalists resisted and attempted to retain hegemony over their units. This proved impossible when the government made the decision to only pay and supply militias that subjected themselves to unified command and structure.
Negrin also began appointing members of the Communist Party (PCE) to important military and civilian posts. This included Marcelino Fernandez, a communist, to head the Carabineros. Communists were also given control of propaganda, finance and foreign affairs. The socialist, Luis Araquistain, described Negrin's government as the "most cynical and despotic in Spanish history."
By June 1937, the Socialist Party had 160,000 members. The growth in the Communist Party was even more dramatic which now had nearly 400,000 members. The communists also controlled the Union General de Trabajadores (UGT), the Catalan Socialist Party (PSUC) and the PSOE youth movement, Juventudes Socialistas Unificadas (JSU).
On 27th February, 1939, the British prime minister, Neville Chamberlain recognized the Nationalist government headed by General Francisco Franco. Later that day Manuel Azaña resigned from office, declaring that the war was lost and that he did not want Spaniards to make anymore useless sacrifices.
Juan Negrin now promoted communist leaders such as Antonio Cordon, Juan Modesto and Enrique Lister to senior posts in the army. Segismundo Casado, commander of the Republican Army of the Centre, now became convinced that Negrin was planning a communist coup. On 4th March, Casedo, with the support of the socialist leader, Julián Besteiro and disillusioned anarchist leaders, established an anti-Negrin National Defence Junta.
On 6th March José Miaja in Madrid joined the rebellion by ordering the arrests of Communists in the city. Negrin, about to leave for France, ordered Luis Barceló, commander of the First Corps of the Army of the Centre, to try and regain control of the capital. His troops entered Madrid and there was fierce fighting for several days in the city. Anarchists troops led by Cipriano Mera, managed to defeat the First Corps and Barceló was captured and executed.
Jesus Hernández went to the Soviet Union and became an executive member of Comintern. He soon became disillusioned with the rule of Joseph Stalin and went to live in Mexico. In his memoirs published in 1953, Hernández admitted that he was following orders from Stalin to oust Francisco Largo Caballero and to get him replaced by Juan Negrin. He also claimed that Stalin did not really care about the Republicans winning the Spanish Civil War and was more concerned with blocking German influence in the country.