At the ballot box, every citizen is equal: rich and poor, capitalist and worker, man and woman. Each has a vote, and no more influence than sits in the vote, and all votes count the same. Then and there, for a moment, power is equalised.

The logic of equality is commanding. It is when citizens are equal politically that a system can be democratic. To the degree that there is political equality, the political agenda reflects the balance of opinion and of interests in citizenry. To the degree there is political inequality, special interests will distort the agenda.

Political equality coexists with economic and other forms of inequality. Away from the ballot box, the rich and the poor are all but equal, nor are the capitalist and worker or men and women. The professor who has a reputation to lean on, the ability to write persuasively and access to space in high-minded newspapers or websites, has more influence than the immigrant who is just passably literate in his adopted language. Once you step out of the voting station, you step back into a land of unequal power.

But the fact of manifold inequalities, even rampant inequalities, does not mean that there is no political equality. In free and fair elections, there is equality of opportunity and impact. Under a democratic constitution, people have equal rights (if not necessarily equal ability to make use of rights). Under the rule of law, people are equal before the law (if not necessarily equal in their ability to work the law to their advantage).

The situation of the humble citizen is vastly different in any democratic system from any autocratic system.

The coexistence of political equality and other inequalities is unavoidably uneasy. Near to a century ago, Justice Louis Brandies of the United States Supreme Court warned: “We may have democracy, or we may have wealth concentrated in the hands of a few, but we can’t have both.” That was at a time of economic crisis combined with extremes of inequality in wealth. But he was wrong. Democracy in America survived, possibly because of political responses in the policies of the New Deal to excesses of economic inequality. Economic inequality is a strong force in society, but so is political equality.

Could economic inequality reduce political equality to irrelevance? It would seem that the answer in the first instance is, no. Where democracy is established and has taken root, the fact of economic inequality does not in itself turn political equality into an empty shell of formality. However, it would also seem that a high level of economic inequality combined with other conditions could make political equality redundant.

In an elegant book on economics and politics titled Equality and Efficiency: The Big Tradeoff, published in 1975, the economist Arthur Okun gave the relevant conditions the name of “transgression.” Economic inequality is not a threat to political equality by its mere existence but becomes a threat if economic power is allowed to transgress from markets, where it has a role, to politics, where it by democratic principles should have no part.

The crude mechanism of transgression is corruption. If money is allowed to buy public policy, political equality is reduced to a pretence. A contributing cause to democracy not taking hold in Russia after the fall of the Soviet Union is entrenched corruption.

The sophisticated mechanism of transgression is to use economic power to usurp political power in ways that may not be technically corrupt or illegal but which nevertheless destroy the impact of political equality. The increasing sway of private money in American politics in recent years is of this kind. Politics have become mega-expensive – actually have deliberately been made mega-expensive for the purpose of making money the ultimate political resource. Candidates and representatives cannot (mostly) hope to win or retain office without raising large amounts of money from sponsors and have thereby become more beholden to the givers of money. Sponsors are now organised in PACs, super-PACs and otherwise and are increasingly able to decide who will be elected – those the money is willing to sponsor – and what policies they can promote and support when elected – those that are acceptable to the money. Furthermore, monied interests have added organisational power to their already formidable economic power and have become able to more decisively dominate language, agendas and discourse.

President Barack Obama used his final State of the Union Address in January 2016 to warn against “democracy grinding to a halt.” In Washington, he explained, elected representatives are “trapped” by the “imperative” of raising money, which pulls everyone into the “rancor” of having to outshout each other. “Democracy breaks down when the average person feels their voice doesn’t matter; that the system is rigged in favor of the rich or the powerful or some special interest. Too many Americans feel that way right now.”