Democratic presidential candidate and former Colorado Gov. John Hickenlooper was loudly booed Saturday when he told California Democrats that “socialism is not the answer.”

Hickenlooper made the unpopular comment at the Democratic Party State Convention in San Francisco.

“Let me be perfectly clear,” he told his not-very-receptive listeners, some of whom were holding signs backing the bid by Sen. Bernie Sanders (I-Vt.) for the Democratic nomination. “If we want to beat (President) Donald Trump and achieve big, progressive goals, socialism is not the answer.”

As the booing from some 4,500 delegates and observers continued over his speech for several seconds, he warned: “You know, if we’re not careful, we’re going to end up helping to re-elect the worst president in American history.”