POUM stands for Partido Obero de Unificacion Marxista which translates into Engish as The Workers Party of Marxist Unification. It was a Spanish political party that was formed by Andres Nin and Joaquin Maurin in 1935 and it was most active from this date until December 1936.

The POUM was strong in Catalonia, although in most other areas of Spain it made little impact. Before the Spanish Civil War estimates of the POUM's size vary from 3000 to 8000. Like most of the left groups it grew during the Civil War and by September 1936, it was around 30,000 strong, with 10,000 in its own militia. Although the Popular Front Government and the Civil War contributed to the party's growth, it also had a negative impact in exposing the centrist politics of the POUM leaders. The POUM was capable of left criticisms and consistently refused to carry through a break with the leaders of the CNT and UGT. Fearful of 'isolation' from these leaders they diplomatically refused to be critical of their practice. They acted as a 'loyal opposition' in the Popular Front, often arguing against the PCE's proposals but accepting to abide by them and even taking responsibility for them when they were defeated. Trotsky called the POUM 'the chief obstacle on the road to the creation of a revolutionary party'. However, unlike Stalinism which refused to adapt to the revolutionary impulses of the masses after July 1936, the POUM wanted revolution, but its desire for revolution went alongside a desire to cover-up the weaknesses and betrayals of the anarchist, socialist and even Stalinist leaders.

An example of the POUM's centrism can be found in its attitude to the Popular Front itself. Before the February 1936 elections the POUM campaigned against any coalition with the republican bourgeoisie. Then, on the very eve of the elections, they actually entered the Popular Front - only to renounce it again when the elections were over. However, Nin's criticism of the Popular Front after February was not that it tied the workers' organisations to the programme of the bourgeoisie but that it was not genuinely a Popular Front. La Batalla (the POUM's paper) of 17 July 1936 on the eve of the Civil War, called for 'an authentic government of the Popular Front, with the direct participation of the Socialist and Communist parties'.

Yet, when the Civil War erupted and the initiative was with the masses, the POUM shifted direction sharply and gave voice to the demands of the socialist revolution. In those early weeks the POUM exercised the leadership in the Lerida revolutionary committee. It was the only committee in Catalonia to refuse to have a representative of the republican bourgeoisie on it. However even here the POUM stopped halfway. It could have used its revolutionary influence in towns like Lerida and Gerona to agitate for the formation of district and provincial Soviet-type bodies which would have developed into a decisive challenge to the authority of the Generalidad. Not only did they refuse this road but Nin went out of his way to explain at great length that Soviet-type bodies were unnecessary and 'alien' to Spain. This rationalisation for the prejudices and libertarian localism of the anarcho-syndicalist masses was typical of the POUM. The POUM tried at every turn of events to minimise the differences and above all to conciliate with the leaders of the CNT.

Nin was to get his wish for a 'genuine' Popular Front in September 1936. Up until 7 September La Batalla denounced 'bourgeois ministers', unlike the PCE which heaped praise upon them. But once the Caballero cabinet was formed in Madrid and the offer was made to the POUM of a seat in the provincial government in Catalonia, all this ceased.

In its place Nin assured the readers of La Batalla that a revolutionary orientation was 'assured' whenever there was a majority of 'socialists' in the government. Nin went so far as to define the dictatorship of the proletariat as a united front of workers' parties and trade union leaders who assume governmental power.

Once the POUM took its seat in the Catalan government it also took responsibility for the measures of the government. Of course, the POUM proposed radical measures to its Stalinist and bourgeois allies such as an industrial and credit bank, and no compensation to factory owners, but these were rejected and the POUM remained respectfully silent. Furthermore, when the government proposed that there should be a government agent in each factory, or that there should be no further elections of factory councils for two years, the POUM agreed. Nin was also willing to accompany President Companys on a tour of Lerida to convince the workers that the powers of the revolutionary committees should be dissolved. Having performed these valuable services for the bourgeoisie, on 16 December 1936 Nin was ousted from the government and the POUM's usefulness was effectively at an end.