For as long as I've been alive, one of the great asymmetries of American politics has been that mainstream Republican Party politicians proudly call themselves "conservative" whereas Democratic Party standard-bearers are reluctant to wear the "liberal" label.
But this chart from the Pew Center indicates that the time may be right for a change — with self-identified liberals now outnumbering self-identified moderates among rank-and-file Democrats, we should expect to see more Democratic elected officials coming out as liberals.
You could easily imagine this becoming a spiral of liberalism, since politicians' reluctance to self-identify as liberal was both a cause and a consequence of the term's unpopularity among the mass public. A generation or two of Democrats have "learned" from the likes of Bill Clinton and Barack Obama that you can favor stricter environmental and public health regulation, higher taxes, and a more generous welfare state without necessarily being a liberal. But if more people call themselves liberals, then more politicians will call themselves liberals too. Which will "teach" more constituents that, yes, they are liberals after all. And that, in turn, will further drive change in how politicians self-identify.