How can Bernie speak for Democrats?

I am struck by the media’s continuing infatuation with Sen. Bernie Sanders, the independent from Vermont.

They keep asking him how Democrats can win elections, how Democrats can topple Republican opponents, how Democrats can do this or do that to benefit their party.

Hold on! Sanders isn’t a Democrat. He caucuses with Democrats in the U.S. Senate. However, he doesn’t belong to a political party.

Is this guy the right man to speak for a party of which he is not a member?