Taking as a starting point the idea that social classes exist "objectively" - that is, as Marx asserted, that a number of individuals occupy a similar position vis à vis others in the capitalist system and that these groups have fundamentally opposing interests in the distribution of the fruits of labour - a fundamental problem for Marxists has been to explain why individuals do not necessarily act on the basis of this common class position. Marx suggested himself that the "superstructure" of society - the ideological, religious, political, and cultural forms of consciousness through which men and women experience life under capitalism can potentially mask true economic interests. For Marx, however, class consciousness matters little in the long run: the capitalist system being inherently contradictory and crisis-prone, the only possible outcome is that individuals will discover their own position within the system and adopt political positions consistent with their social class. As the history of capitalism has, so far, not played out according to Marx's predictions, other Marxist authors have proposed alternative explanations.
Some, like Antonio Gramsci have placed more emphasis on the role of ideology in preventing the working class from acting in accordance with its interests. The Italian thinker and politician proposed that the ideological foundations of capitalist society lied in the specific actions of the ruling class (capitalists and their allies), who develop and impose a system of ideas to justify their rule. Part of the class struggle thus has to do with the battle to develop a genuinely working-class culture and politics, as economic forces alone cannot guarantee that the proletariat will take political actions consistent with its own interest. Lenin, in What is to be done? held a slightly different view: workers can defend their own interests by forming unions or other class-based organizations, but these initiatives only have the limited aim of improving immediate conditions or defending the position of certain segments in the working class and do not naturally lead to political revolution. For Lenin, the revolution could only be brought about by a dedicated group of intellectuals and politicians, a "vanguard" of the working class to coax its members into action.
In more practical terms, the UK serves as an interesting example of changes in the way the working class has voted during the 20th century. The rise of the Labour Party before 1945 appears to follow the path predicted by Lenin and Marx, albeit in a non-revolutionary manner. While the Labour Party only received a fraction of the votes (1.3%) in the 1900 general election, this figure rose to 37.1% in 1929, ushering in the first British Labour Prime Minister. In 1945, support for Labour reached its peak, with 47.7% of the electorate voting for Clement Atlee's first post-war government. Since then, Labour has remained the largest or second-largest political group in the House of Commons, but the party has itself moved away from its working class roots. For example, between 1969 and 1987, support for Labour amongst manual workers declined from 69% to 45%. What explains this change? An early diagnosis was the "embourgeoisement thesis", which suggested that as the working class becomes wealthier, its members increasingly identify as middle class. There is at, best mixed evidence for this view. Another possibility is that Labour is no longer the party of the working class nor is it on the Left. In contrast, other authors have argued that Labour has remained on the left but the views of the working class on morality no longer properly align with modern social-democratic politics - the latter explanation being consistent with most of the anti-immigrant, pro-Brexit positions. Indeed, there is evidence that the proportion of individuals who identify as working class has remained stable since the 1980s in the UK. Finally, sociologists might also suggest that the working class itself has vastly changed in scope and nature, with most jobs now being concentrated in the service sector, where class consciousness is less likely to emerge due to "individualized career paths" and where it is more difficult to form trade unions.
I want to conclude by approaching critically the question of class interest. As the Marxist sociologist Eric Olin Wright has argued, class conflict not only takes place over efforts to change the entire game (ie. capitalism vs communism), but can also be about changing the rules of the game, or making moves within the game. Individual workers might thus believe they are better off under a capitalist system or a conservative government if they perceive social mobility to be high, and members of a trade union may benefit from their bargaining position in the labour market and wish to protect their arrangements at the detriment of the working class at large. Identifying class interests is thus not a clear-cut procedure; unless one is able to demonstrate clearly how the working class is better off under a specific political system or set of policies. Finally, it may simply be that individuals, or even members of the working class, do not always define themselves primarily in terms of class. For Anthony Giddens, there are competing sources of identity formation or "structuration" in Western societies, which lead individuals to often vote on the basis of their gender, their race, their nation or their family ties, sometimes entirely against their own class interests. The decline of social ties formed on the shop floor and rise in friendships based on common interests through social media are examples of changes in the process of "structuration".