Richardson, who unsuccessfully sought the Democratic presidential nomination in 2008, said Hillary Clinton is the pro-business candidate who can win the White House in 2016.

While Sanders has pushed the party to the extreme left, Clinton will bring the message back to the center during the general election, said Richardson. He supported Barack Obama for president in 2008 over the former first lady, who later became Obama's secretary of state.

"Clinton is going to be somebody, I think, the American people see as moderate," Richardson said. "The country moves to the center in a general election."

Dear Sisters and Brothers -

Sunday, at the grand opening of the Broward County Democratic Party's new headquarters, I spoke in a crowded room with Debbie Wasserman Schultz. I asked if she had read the letter we had sent last week. She said she had not read any letter from our campaign, even though we had sent it to three of her addresses, including her campaign headquarters, her DWS PAC, and her office at the Democratic National Committee.

I then produced a copy and said “I was happy to hand deliver it.” She refused to take the letter and told me to give it to one of her staffers. I said I would, and I told her the letter was proposing a series of debates throughout the district. I asked her if she would agree to scheduling debates. She asked how we could have debates when no one has even filed yet. I reminded her of what she said recently about the Democratic presidential debates on The Daily Show, that she would want the candidates to debate every day, and I'm proposing a lot fewer than every day. She cut off the discussion without agreeing to a single debate.

Later, when I was leaving the party, I noticed her staffer standing near a doorway. I thought, well at least I can hand her staffer the letter proposing a series of debates. But she refused to accept the letter, she said she had “nowhere” to put them. I suggested she put them in one of the two huge pocket bags she had over each of her shoulders. She said there was no room and that she had read the letter already.

That’s fine, for now. If the Wasserman Schultz campaign wants to play games and lash out at me and my staff, then we’ll stay focused on talking to the voters about the progressive vision we have for this country.

"My response is if you want to question my qualifications, then maybe the American people might wonder about your qualifications Madame Secretary," he said.

Sanders added: "When you voted for the war in Iraq, the most disastrous foreign policy blunder in the history of America, you might want to question your qualifications. When you voted for trade agreements that cost millions of Americans decent paying jobs, and the American people might want to wonder about your qualifications. When you're spending an enormous amount of time raising money for your super PAC from some of the wealthiest people in this country, and from some of the most outrageous special interests ... Are you qualified to be president of the United States when you're raising millions of dollars from Wall Street whose greed and recklessness helped destroy our economy?"