Richard Reeves sums up at Brookings:
The top quintile has been pulling away from the middle class, even without the top 1 percent in the mix. But not from the bottom quintile, according the CBO. In fact, income growth for the poorest group has been as great as for the upper middle class (79 percent versus 78 percent growth between 1979 and 2015), while the middle class have experienced sluggish growth by comparison (46 percent). As Bob Samuelson points out in the Washington Post, it is simply wrong to claim that has been no increase at all for middle class households—and it is unhelpful when that claim is made. Low growth is better than no growth; but it is still low.
It hardly needs saying that this does not mean all is well for those on the lowest rung of the ladder. These are measures of growth, not level, and 79 percent of not very much is still not very much.