SAN FRANCISCO — John Hickenlooper has been an afterthought in the Democratic presidential race, but his tough talk Saturday to thousands of party activists in an important primary state caught him immediate attention — and a steady chorus of jeers.

“We know that it is absolutely essential to beat Donald Trump — but it is not sufficient,” the former Colorado governor said here at the California Democratic Party convention. “We must address the divisions tearing us apart. We must tackle the kitchen-table challenges facing Americans. But let me be clear, if we want to beat Donald Trump and achieve big progressive goals, socialism is not the answer.”

Activists in the crowd erupted with loud boos, disrupting Hickenlooper for several seconds. They booed again when he argued against eliminating private health insurance and against trying “to tackle climate change by guaranteeing every American a government job."

Hickenlooper introduced this line of attack against the left wing of his party earlier this week, in interviews with NPR and MSNBC’s Rachel Maddow. Escalating the attack at the state party convention — a forum where “Bernie!” chants rang out with great frequency and where a sea of waving Tulsi Gabbard signs greeted attendees descending the escalators — was a purposely provocative move from a candidate registering around 1% or less in most polls. His campaign circulated the choicest excerpts hours before he took the stage at the Moscone Center.

“As Democrats, if we don’t define a clear space between us and socialism, we run the risk of helping to reelect the worst president in American history, and we should take that very seriously,” Hickenlooper told BuzzFeed News in an interview after his remarks. “The Republicans are definitely already trying to paint us into a corner as socialists, and we need to be very clear to say we are not socialists. We believe we can create a better country, with a better future, but it’s not through massive [expansions] of government.”

As for Medicare for All, which has emerged as a rallying cause for progressives, Hickenlooper pledged in his speech to “make health care a right, not a privilege, with a real public option” to buy government-run insurance — an option that falls short of what many activists prefer.

“I don’t mind a single-payer system,” Hickenlooper told BuzzFeed News. “But I don’t think we can create that change by edict. It just doesn’t work that way in this country.”