Barack Obama calls inequality the "defining challenge of our era". Polls show that a majority of Americans now believe that inequality has grown over the past decade and favour tax increases on the wealthy to help the poor. The non-partisan Pew Research Centre recently found that six out of 10 Americans believe their economic system unfairly favours the wealthy.

What's the reaction of conservatives? They want to change the subject. Those with presidential ambitions say we should focus on poverty rather than inequality. Senator Marco Rubio of Florida points to the "lack of mobility" of the poor as the core problem. Representative Paul Ryan of Wisconsin blames their isolation from mainstream America. "On every measure from education levels to marriage rates, poor families are drifting further away from the middle class."

Conservative New York Times columnist David Brooks argues that the "interrelated social problems of the poor" have nothing to do with inequality. Even some Democratic operatives are worried that talking about inequality will turn off voters. "However salient reducing income inequality may be," writes Mark Mellman, a Democratic pollster, "it is demonstrably less important to voters than any number of other priorities" – including reducing poverty.

This is rubbish. Widening inequality is making it harder for the poor to escape poverty and thwarting equal opportunity. Let me explain. When almost all the gains from growth go to the top, as they have for the last 30 years, the vast middle class doesn't have the purchasing power necessary to keep the economy growing and generate lots of jobs.

Once the middle class has exhausted all its coping mechanisms – wives and mothers surging into paid work (as they did in the 1970s and 1980s), longer working hours (which characterised the 1990s) and deep indebtedness (2002-2008), the inevitable result is slower growth and fewer jobs.

Slow growth and few jobs hit the poor especially hard because they're the first to be fired, last to be hired, and most likely to bear the brunt of declining wages and benefits.

A stressed middle class also has a harder time being generous to those in need. It's no coincidence that the tax revolts that began thundering across America in the late 1970s occurred just when middle-class wages began stagnating. Helping America's poor presumably requires money, but the fiscal cupboard is bare – and the only way to replenish it is through tax increases on the wealthy because the middle class is stretched to the limit. The shrinking middle class also hobbles upward mobility. Not only is there less money for good schools, job training and social services, but the poor face a more difficult challenge moving upward because the income ladder is far longer and its middle rungs have disappeared.

American conservatives also don't want to acknowledge any connection between widening inequality and unequal political power. Brooks, for example, warns that any discussion of unequal political power will make it harder to reach political consensus over what to do for the poor.

But it's precisely the concentration of power at the top – which flows largely from the concentration of income and wealth there – that has prevented Washington from dealing with the problems of the poor and the middle class. As wealth has accumulated at the top, Washington has reduced taxes on the wealthy, expanded loopholes that disproportionately benefit the rich, deregulated Wall Street and provided ever larger subsidies, bailouts and tax breaks for large corporations. The only things that have trickled down to the middle and poor, besides fewer jobs and lower salaries, are public services that are increasingly inadequate because they're starved of funds.

Unequal political power is the endgame of widening inequality – its most noxious and nefarious consequence. Big money has all but engulfed Washington and many state capitals – drowning out the voices of average Americans, filling the campaign chests of candidates who will do their bidding, financing attacks on organised labour and bankrolling a vast empire of rightwing thinktanks and publicists that fill the airwaves with half-truths and distortions.

The final reason conservative Republicans would rather talk about poverty than about inequality is because they can then characterise the poor as "them" – people who are different from most of us, who have brought their problems on themselves, who lack self-discipline or adequate motivation. Accordingly, in their view, any attempt to alleviate poverty requires that "they" change their ways.