WASHINGTON (MarketWatch) — America’s biggest economic challenge is also its greatest political problem. The widening gulf between those who have it all and those who have almost nothing threatens not only our economic prosperity but also our cohesion as a nation.

While the billionaires and millionaires have more political clout than at any time in the past 100 years, those at the bottom who are scraping to survive are increasingly disconnected from political life.

“ Big money sets the agenda, affects how the campaign develops, and shapes how particular people and policy problems get defined.” ” — --Darrell West

Five years after the Supreme Court legalized unlimited spending on political campaigns, the takeover of the Republican Party by the Koch brothers and other billionaires is nearly complete. The Kochs have announced that they and their network of wealthy donors will spend nearly $900 million in the 2016 election, nearly as much as the two parties will spend.

The Kochs and other billionaires couldn’t buy the 2012 presidential election, in part because Mitt Romney couldn’t disguise the fact that he was a plutocrat himself. But they had much more success in the 2010 and 2014 off-year elections, where their money went a long way toward electing a very conservative, Republican-controlled Congress.

Big money can’t always buy the outcome it desires. But as the Brookings Institution’s Darrell West writes: “During a time of rising campaign costs and limited public engagement in the political process, big money sets the agenda, affects how the campaign develops, and shapes how particular people and policy problems get defined.”

“Billionaire activism very well could tilt a close election in favor of conservative interests,” West concludes.

The story of billionaires and politics is nothing new. What’s less appreciated is how little power is wielded by those at the opposite end of the spectrum.

Capital Journal: The GOP and 2016

Conservative thought is haunted by Tocqueville’s prediction that unfettered democracy will inevitably bankrupt a nation because the majority will vote for ever-larger welfare benefits for themselves that are paid for by a minority of wealthy taxpayers. As more people depend on government, they’ll vote themselves higher and higher benefits until most people are sponging off the few remaining taxpayers. Collapse ensues.

That nightmare is far from reality. In fact, those who are most dependent on government benefits are the least likely to vote, while the people most likely to participate in politics are those who feel financially secure without the helping hand of food stamps, Medicaid and TANF.

According to the Pew Research Center, voter turnout in 2014 for the one-fifth of the population who were most financial secure was about 63%, and turnout was 51% for the next most-secure fifth. But participation dropped off sharply for other groups. Turnout fell to just 20% for the 60 million people who are most financially insecure.

“ We can’t have citizens who are fully engaged politically unless we also have citizens who are fully engaged economically. ”

The people who could, theoretically, vote themselves bigger welfare checks don’t actually vote. Meanwhile, the people who want other things from government, such as low taxes on capital income, or easy regulations on the coal companies and banks that they own, are heavily engaged in the political process, from giving money to voting to meeting directly with officials.

Support for the Republican Party followed a familiar pattern: The most financially secure fifth were the most likely to vote Republican (49% of this group favored Republicans, while 42% favored Democrats). Support for the GOP fell as financial security faded; just 17% of the least financially secure supported the Republicans in 2014. That matches what most people think: The rich are more likely to be Republicans.

But contrary to conventional wisdom, poor people are not more likely to support Democrats than rich people are. Nearly equal numbers — about 40% to 43% — of every quintile supported Democrats in 2014.

Democrats lost the congressional election in 2014 not because they had less support, but because their supporters — many of them in the working class or in poverty — didn’t vote.

This isn’t just a complaint that Democrats didn’t get the vote out; it’s an observation that participation in politics and civic life declines sharply as people suffer from financial stress. If the middle class can’t find some greater security (such as higher incomes, better job prospects and greater opportunities for their children to get ahead), they’ll begin to behave more and more like the very poor, who are hardly engaged at all.

That would be bad for our democracy, because it would mean that the views of the wealthy and their allies in the management class would hold even greater sway in Washington, state capitols and city halls. Already, our politicians pay far more attention to the concerns of the wealthy. That’s why our political class obsesses about deficits while unemployment lingers, and why efforts to address climate change are blocked. It’s why we can’t reform a tax break that primarily benefits people who make over $200,000.

And that’s why we see these conservative groups working overtime to make it harder to vote, even as they celebrate their own freedom to buy elections. And that’s why they rejoice when gridlock in Washington discourages more people from believing in the power of collective action.

The disengagement of the middle class could pose an even greater threat. People who are disengaged from politics are more susceptible to being persuaded by demagogues who take advantage of their anger and ignorance.

Our democracy needs informed and engaged citizens of all stripes. But it seems that we can’t have citizens who are fully engaged politically unless we also have citizens who are fully engaged economically.