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.