Bernie Sanders fought back against Clinton's idea that he's only a single issue candidate when he was asked about it by CNN's Jake Tapper:

TAPPER: Hillary Clinton has been referring to you as a single -- you know that, sir. Hillary Clinton has been referring to you as a single- issue candidate. What do you think when she says that?

SANDERS: I haven't the vaguest idea what she's talking about. If she thinks that income and wealth inequality, and the fact that the rich get richer while everybody else gets poorer is the only issue, it's not.

We're the only major country on earth that doesn't have national health care system, guaranteeing health care to all people, talk about that. We need to have free tuition and public colleges and universities so that our kids today can do well in a global economy. We need to have the wealthiest people in larger corporation pay their fair share of taxes. We're working very hard to transform our energy system so we effectively combat climate change. We are talking about dozens of issues so I'm not quite sure where Secretary Clinton is coming from.

But what I will repeat is I think we have got to address this real crisis of a corrupt campaign finance system in which billionaires and their super PACs are trying to buy elections. Now, one of the things that we're up against right now, Jake, is that Hillary Clinton's super PAC funded by Wall Street and other powerful special interests are going to pour a lot of money into this election against us.

We rely on individual contributions, 27 bucks a piece. So you're seeing the impact of billionaire and Wall Street super PACs on an election. That has got is to change. We have to overturn this disastrous Citizens United Supreme Court decision.

Bernie's power has come from his economic populism and he's tying income inequality to many of the problems that Americans are facing today.