The basic formula of capitalist production: M-C-P-C’-M’
Money buys capital, both constant capital such as buildings, machinery & raw materials, & variable capital, i.e. labour power, & in the act of production produces commodities that are sold for a value greater than the initial amount of money originally started out with.
Crises are possible at each point in the process.
Underconsumptionists emphasise the C’-M’ process (problems realising SV), whilst profit squeeze theorists the M-C process (problems producing SV).
The credit system allows overproduction to develop on a large scale by separating the act of purchasing from the act of paying.
If not enough labour power then industrial capitalists will hoard money, giving the appearance of a shortage of money & an overproduction of commodities. But the essence of the crisis is not a problem realising surplus value, but producing surplus value due to a lack of labour.
Marx called this a crisis of ‘absolute overproduction of capital’.
In the real world a crisis of relative overproduction is likely to break out before a crisis of absolute overproduction.
Class-struggle theory of Peter Bell & Harry Cleaver argues that the crisis takes place at the productive phase as workers more & more resist their exploitation by capital.
“…the rate of accumulation is the independent variable, not the dependent variable; the rate of wages, the dependent, not the independent variable.” Marx, Capital, Volume I, Part VII
The 2007/08 crisis seems to be nothing to do with a shortage of labour as a result of class struggle pushing up wages.