Hickenlooper, a former governor of a purple state Hillary Clinton won by just 4.9 percentage points in 2016, said Republicans would use the Democratic flirtation with socialism to paint the entire party as outside the mainstream of the political spectrum.
"If we want to beat Donald Trump and achieve big progressive goals, socialism is not the answer," Hickenlooper told delegates to the California Democratic Party convention.
Some delegates, meeting in perhaps the most liberal city in America, booed.
Hickenlooper, polling near the bottom of the 24-candidate Democratic field, anticipated the reaction. He amassed a centrist record as governor, working with a Republican-controlled legislature for most of his two terms in office, though he scored progressive wins in his final two years on the job after Democrats reclaimed control of the state House and Senate.