Hey Bernie, cool it on Hillary, warns this former Democratic hopeful

Bill Richardson: This is what's fueled Donald Trump...
Bill Richardson: This is what's fueled Donald Trump...

Bernie Sanders needs to soften his verbal assaults on Hillary Clinton if he wants to stay relevant beyond the 2016 election cycle, onetime Democratic presidential hopeful Bill Richardson said Thursday.

Richardson told CNBC's "Squawk Box" he believes Sanders will stay in the Democratic race for the White House to this summer's convention, even though he doesn't have a chance of winning, so he can give further voice to his hot-button issues of wealth inequality and Wall Street reform.

"I think [Sanders] realizes he can't win. But at the same time, he wants to be a player in the party. He wants to influence the platform," said Richardson, former governor of New Mexico and an official in President Bill Clinton's administration. "If he's going to do that, I think he needs to start moving in the direction of supporting the nominee."

After losing four out of the five state primaries Tuesday, Sanders said he's laying off hundreds of campaign workers. But a spokesperson for the Vermont senator played down the move, saying with just 10 states left they need fewer people.

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."