Here's the chart.
During the time span of this chart, Democrats held the presidency for 10 of the 18 years.
So it's worth noting that simply electing Hillary! not only won't change things because Hillary! is not a populist, not on the side of the middle-class or lower-class, and in fact doesn't understand the concept of irony, given that she recently wore a jacket that cost $12,495.
She wore it while she gave a speech about income inequality.
But the real problem is that people focus on the presidency. During the time charted above, Democrats DEMOCRATS had control of the Senate for only 6 years (and one 2-year session was a 50-50 tie) and of the House of Representatives 4 of 18 years.
In the 1990s, the Democrats had control of both the House and the Senate for 6 years (1989-1995). Republicans were in charge the next 6, controlling both houses. From 1990-2000, Democrats controlled the presidency for 8 of 10 years.
During the 1990s, median household income overall rose in 49 of 50 states. In fact, household income rose steadily whenever Democrats were in charge. The charts where I got that data from show only overall household income, not broken down by bracket, so it's possible that the median income was rising simply because the wealthy were getting richer, but that's not exactly how medians work. (That's how AVERAGES work.)
With a median, if we take four people, with incomes of $1,000, $5,000, $5001, and $100,000, the median is $5,000.50, while the average is $27,750.
If the highest earner's income in that sample increases to $2,000,000, the median doesn't change. But the new average income is $502,000. In each case, the average misrepresents the real state of income, which is why averages are a horrible measurement. They tell you nothing.
The median changes only slightly, when there is a change near the middle. In our four-person example of $1k, $5k, $5001 and $100k, giving the $5001 guy a raise to $10,000 changes the median to $7,500. (In even-numbered sets the median is determined by averaging the two numbers at the center.) If you add a fifth person to this new set, making $200,000, the median drops back down to $5,001 -- the point where the number of people on each side are the same.
So it's hard to move a median up very high just by adding to one end.
Anyway, the real point is this: Congress controls the purse strings. Congress sets tax policy. Congress determines write-offs and loopholes. Congress determines that most hedge-fund investors pay a maximum income tax rate of 20% on their earnings from those investments. (Which helps make the effective income tax rate on the highest earners a relatively nominal 23% or so, having fallen from 27% since 1979. As with incomes, the effective tax rates on the highest earners showed changes while the Democrats held Congress; the effective tax rate fell steadily since 1979 except for brief periods in the 1990s and 2000s more or less coincident with Democratic control of Congress -- and the latter period was under the Worst President Ever's administration.
Presidents can reject laws, and can be choosy about how to enforce them, but they cannot unilaterally change tax rates or wages or allocate more money to small business loans or change how student loans are collected and funded.
The Republicans have for nearly 50 years set their main strategy on getting control of legislatures. The Democrats focus on high-profile presidential races. The Republicans control 2/3 of all state legislatures. Those legislators set state tax policy, control school funding, control state business grants and frequently administer or allocate federal funds. They also set the district maps for Congressional races, which is how the Republicans have made Congress so effectively theirs for a long time.
Democrats consistently fail to vote in mid-term elections. 1/3 of Congress is elected at a mid-term election.
Yes, yes, Hillary! helped further crack the glass ceiling. We elected the first African-American president 8 years ago and America is no better for it. Hillary! will not help, not just because she has no interest in changing the economic status quo, but also because Democrats aren't likely to change the legislative makeup this year. The Democrats would have to win 19 of the 27 state legislatures that are at risk of changing in 2016; 11 of the 27 are deemed relatively safe for the GOP. So the Democrats will have to win the 16 that are generally considered gettable, and at least 3 of the ones nobody thinks the GOP will lose.
Democrats need to get 30 extra seats in Congress to take the House. in 2014, they got 15. The 188 Democrats sworn into the House after the 2014 midterms was the lowest the party has gotten since 1947.
Until Democrats bother voting for something other than president, the US will continue becoming more unequal.