Liberals in the modern political sense constitute about 19% of the adult citizenry in America. This a core constituency of the Democratic Party, but not its only group, and not always reliably Democrat, as votes for Green Party candidates are drawn almost entirely from this group. Several Democratic subgroups have fewer children than average, but this is the sector which most dramatically pulls that party's average down.
This is also the sector of the population which most strongly supports the transformative, and very expensive, aspects of the bailout bill, the stimulus package, and the federal budget increases. One of the warnings about these jaw-dropping amounts of funds that conservatives, and especially libertarians, keep bringing up is the amount of debt we are foisting off onto future generations - usually phrased as "our children and grandchildren."
So why would progressives care about them, exactly?
It's a rather ugly thought, I admit. But there it is.