Howard Dean, a former Democratic presidential candidate and ex-governor of Vermont, issued a stern warning Monday to Sen. Bernie Sanders: Lay off Hillary Clinton, it's only helping Republicans.

"[Sanders] has got to stop with the speeches about Goldman Sachs and all this other stuff," Dean told CNBC's "Squawk Box."

"Bernie is a deeply committed guy with an ideology. He's going to want to go to the convention ... and going to give a speech about his ideals," said Dean, formerly chairman of the Democratic National Committee.

In an interview Sunday, Sanders rejected calls that he leave the race. "We intend to take the fight all the way to California," he said on ABC's "This Week." The California primary is on June 7, the final day of voter contests before the Democratic convention.