Class warfare is returning to the presidential campaign. The major contenders are increasingly making middle-class wage stagnation and the vast earnings of America's super-rich a focal point of their bids for the White House.

Hillary Clinton, the Democratic front-runner, promises to champion the middle class and says that as president she would take steps to reduce income inequality, give everyday people more advantages and cut the power of Wall Street and the super-rich. She favors higher taxes on the wealthiest Americans, an increase in the minimum wage and other programs to help the middle class and the poor.

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But Sen. Bernie Sanders of Vermont, a self-described democratic socialist and Clinton's main competitor for the Democratic presidential nomination, is escalating his already-hot rhetoric in an effort to outflank Clinton as a populist. And he has been attacking billionaire Donald Trump, the Republican front-runner, with more frequency on populist grounds. "We all know people are angry and they are upset for a lot of good reasons," Sanders told an overflow crowd at a high school gym in Worcester, Mass. last weekend. "But what we have got to do is not let Trump and these other people divide us up. Latinos who are picking tomatoes for eight bucks an hour are not the reason that the middle class of this country is disappearing. Muslim kids who are going to schools studying to be engineers are not the reason we have income and wealth inequality."

And emotions run high. At a rally at the University of Massachusetts in Amherst, a man wearing a Trump T-shirt heckled Sanders. The man was escorted out after the crowd booed him and chanted "Bernie! Bernie! Bernie!" Sanders declared, "I say to Mr. Trump and his supporters that the billionaires in this country will not continue to own this nation."

Sanders is calling for an increase in the minimum wage, free tuition at public colleges and universities, guaranteed paid family leave for workers, breaking up big banks and a $1 trillion program to create jobs and rebuild the nation's infrastructure such as roads and bridges.

Trump has labeled Sanders a "wacko." But Trump understands the anger in the electorate and he is taking a populist approach but with a different emphasis. He calls for higher taxes on hedge-fund managers, who are among the richest people in America, but he is mostly going after what he says is entrenched power and stagnant leadership in Washington. In the process, Trump is giving voice to many Americans, mostly white working-class people and those without college degrees, who feel left behind in the economy and ignored by the nation's leaders while the rich and big corporations expand their power and profits. Trump's argument that he will mostly finance his own campaign, so he won't be beholden to special interests, has a special resonance with his core constituencies.

Opinion research confirms the trends. The latest NBC News/SurveyMonkey/Esquire poll finds that 54 percent of whites feel angry about current events more than they did a year ago. Forty-three percent of Latinos say they are more angry as do 33 percent of African-Americans. Overall, 49 percent of Americans say they are angrier than they used to be. And 52 percent of Americans say the American Dream – the belief that an individual could be successful with hard work and adequate opportunity – once held true but no longer does.

"Meanwhile, the dividing line that used to be the most crucial of them all – class – has increasingly become a division within the parties, not between them," writes Republican strategist David Frum in The Atlantic. "Since 1984, nearly every Democratic presidential primary race has ended as a contest between a 'wine track' candidate who appealed to professionals (Gary Hart, Michael Dukakis, Paul Tsongas, Bill Bradley and Barack Obama) and a 'beer track' candidate who mobilized the remains of the old industrial working class (Walter Mondale, Dick Gephardt, Bill Clinton, Al Gore and Hillary Clinton). The Republicans have their equivalent in the battles between Wall Street and Main Street candidates. Until this decade, however, both parties – and especially the historically more cohesive Republicans – managed to keep sufficient class peace to preserve party unity. Not anymore, at least not for the Republicans."

Frum, a former speech writer for President George W. Bush, points out that, "The Great Recession ended in the summer of 2009. Since then, the U.S. economy has been growing, but most incomes have not grown comparably. In 2014, real median household income remained almost $4,000 below the pre-recession level, and well below the level in 1999. The country has recovered from the worst economic disaster since the Great Depression. Most of its people have not. Many Republicans haven't shared in the recovery."

Political scientist Bill Galston, a former White House adviser to President Bill Clinton, wrote in the Wall Street Journal this week, "Americans are confused and worried. They know that the economy isn't working for them the way it once did, but they don't understand why. This opens the door to candidates who can blame specific malign forces – bureaucrats, plutocrats, autocrats, whatever – and offer sound-bite solutions to complex problems."