This remark reveals, I think, a really substantial error in how people think about parties. It's as if they believed the party could be characterized by taking some sort of arithmetic sum or average of the opinions of the people who comprise it.
But this ignores the fact that the party is an institution with a structure, with mechanisms of operation and levers of power -- levers which are in some hands and not others.
Among Democrats, it's the aisle-crossers who control the party as an institution. They're like the tiller on a boat -- an inch this way or that, and you've tacked. Or gybed, as the case may be.
It's true that if you average up the (expressed) views of Democratic and Republican officeholders you end up with two different-sounding songs. But all the Bernie Sanderses and Dennis Kucinich-es and Ted Kennedys etc ad soporem are in effect lashed to a chariot whose reins are firmly in the hands of the Lantoses and Liebermans. So the ineffectual enlightenment of the former is worse than useless -- it's an actual snare and delusion, like the sweet nectar that draws the poor fly into the flytrap.
I like to think of the two parties as being a lot like McDonald's and Burger King. In practice, they're marketing the same thing, but they're going after slightly different demographics and have slightly different marketing and branding strategies, and slightly different Secret Sauces to mask the rancid flavor of the same low-grade beef.