"But either way, the French case illustrates an important point for socialists to remember: the political power of the capitalist class flows not just from what capital can do, but from what it can choose not to do — invest. It is its control over the investment function, not its collective organizations, that is the key source of capitalists’ power in the political sphere: since, in a capitalist economy, investment is the prerequisite for growth, employment, and tax revenue, policymakers will always have an incentive to prioritize the demands of business confidence over all other considerations.
The only alternative is to attempt to seize control over investment. This was not an approach that Mitterrand was, in the end, willing to entertain."
(By Jonah Birch, contributing editor of Jacobin, available here: https://www.jacobinmag.com/2015/08/francois-mitterrand-socialist-party-common-program-communist-pcf-1981-elections-austerity/)
The article makes it quite clear that the political contest between left forces in the capitalist state and capitalist finance in the markets is fought via the mechanisms of investment, all of which capital presently holds in its own hands.
The key questions of a left government that does not want to be defeated by capital are 1) how to wrest control over investment from capitalists, thus controlling currency speculation and restricting capital flight and 2) how to embark on a growth and productivity plan without being thwarted by inflation as consumption heats up?
History has of course seen governments triumph over business when it comes to increasing the social wage, protecting jobs and coercing capital into job creation. The Postwar era in Europe saw the creation of such regimes of "social compromise." However, there is a clear time limit to such Keynesian reform policies. Once the economy is operating at full speed, with employment at its limits and consumption rising, inflation will take off. This weakens the currency and curtails investment as profits are threatened or as capital moves. Moreover, as James Galbraith has observed, the foundations of the Keynesian full employment system have been more or less abolished. Nevertheless there remains some stimulus to private investment (such as QE and lowering inflation rates) followed by the much discussed withdrawal of these props even in contemporary capitalism. This cycle resembles what Michal Kalecki called the "political business cycle":"This state of affairs is perhaps symptomatic of the future economic regime of capitalist democracies. In the slump, either under the pressure of the masses, or even without it, public investment financed by borrowing will be undertaken to prevent large-scale unemployment. But if attempts are made to apply this method in order to maintain the high level of employment reached in the subsequent boom, strong opposition by business leaders is likely to be encountered. As has already been argued, lasting full employment is not at all to their liking. The workers would 'get out of hand' and the 'captains of industry' would be anxious to 'teach them a lesson. Moreover, the price increase in the upswing is to the disadvantage of small and big rentiers, and makes them 'boom-tired.'
In this situation a powerful alliance is likely to be formed between big business and rentier interests, and they would probably find more than one economist to declare that the situation was manifestly unsound. The pressure of all these forces, and in particular of big business -- as a rule influential in government departments -- would most probably induce the government to return to the orthodox policy of cutting down the budget deficit. A slump would follow in which government spending policy would again come into its own."
A reminder from the Postwar era that the objection to full employment policies or policies which seek to increase the living standards of the masses are not objected to on pure, rational, timeless economic grounds but on the grounds of a class logic. When combatting that class logic, however, it will do to remember that it operates levers of material power. That which it does not like intruding in its affairs, capital can swat aside. The only way to win is to break the rules of the capitalist game.