Last month, Democrat Bill de Blasio won the New York City mayoral race by the largest margin for a nonincumbent in the city's history, leaning heavily on a "tale of two cities" message about income inequality.

His landmark victory did not occur in a vacuum. Rather, it reflected a broader shift by the Democratic Party to focus on the growing gap between rich and poor.

The issue reached peak exposure on Wednesday, when the party's leader identified the broadening income gap as the "defining challenge of our time."

"The combined trends of increased inequality and decreasing mobility pose a fundamental threat to the American Dream, our way of life, and what we stand for around the globe," President Obama said in a speech.

For Democrats, the pivot makes sense. Addressing income inequality is a popular position that excites the party base and plays well at the polls. It also represents a major weak spot for Republicans.

While the income gap has been widening for decades, it notably worsened following the Great Recession, such that the top 10 percent of the nation now hauls in a greater share of the country's total income than at any time since the 1920s. While the median household income for all Americans is roughly in line with where it was in 1989, median income for those at the top is on the rise, according to the most recent Census data.

The "very definition of what it means to be middle class is being undercut by trends in our economy that must be addressed," Treasury Secretary Jacob Lew said in September.

It's in this environment that Occupy Wall Street became such a prominent, if short-lived, national movement. Suddenly, there's a larger pool of people who feel they're being squeezed out by an economy that caters to the wealthiest.

More than three quarters of Americans believe that "the rich just get richer while the poor get poorer," according to Pew. Democrats are most likely to agree with that statement (92 percent), but even a majority of Republicans (52 percent) do as well.

Crucially, black and Hispanic Americans — two key demographics Republicans are desperate to win over — overwhelmingly agree with that statement, too. And in a sign of the changing times, young Americans ages 18 to 29 view socialism more favorably than they do capitalism.

That's not to suggest that Democrats are going to go full Marx, but merely to note that Americans are indeed receptive to messages about winnowing the wage gap. Think Obama's comfortable re-election over Mitt "47 percent" Romney, which can be partly attributed to a message about ensuring everyone pays their "fair share." Even Rep. Paul Ryan (R-Wisc.), known best for his radical budget proposals that would disproportionately target lower tax brackets, is now talking about the scourge of poverty.

It helps Democratic politicians that their base is fully aligned with this trend.

Sens. Elizabeth Warren (D-Mass.) and Bernie Sanders (I-Vt.), two of the most progressive members of the upper chamber, held a hearing last month about how income inequality is literally killing people because of its impact on mortality rates among poorer Americans. Warren has also in recent months made a case for a $22-per-hour minimum wage. And at a time when Republicans are preaching entitlement cuts, she has called for dramatically expanding Social Security.

It's no surprise then that liberals have been clamoring for Warren to run for president, which in turn has pushed more centrist potential candidates to the left. Indeed, Hillary Clinton has already begun to address income inequality in speeches around the country.

It also helps Democrats that Republicans continue to be perceived as the party of the rich, an image that hasn't been helped by the GOP's ongoing attempts to slash food stamps. Indeed, Democrats love the issue so much they're going to try to pull off the ultimate magic trick: Shrouding the unpopular Affordable Care Act in the superhero cloak of progressive economics.

Obama used his Wednesday speech to frame ObamaCare in terms of income inequality, building on accusations that the GOP's regressive stance on health care negatively impacts the poor and the middle class. If Obama can successfully tie the issues together, then you can bet income inequality will be featured even more prominently as we head toward the 2014.