Leftists have always disdained the term "liberal." Liberals are those who support the establishment and want to tweak it. They don't want to over turn it, they don't want to change the system, and they don't want to take away the power of the elites from them. Instead they just want to make it gentler, kinder, and not so harsh, so the victims of the hegemony are not quite as victimized.
People to the left of the liberals didn't go to the term progressive out of shame of being seen as too liberal. They did so out of disgust of being associated with the weasel liberals like Humbert Humphrey who got himself on the national stage by purging leftists in Minnesota.
But Clintonistas and their corporatist ilk thought they could triangulate not only the right with their policies but the left with their terms and started calling themselves progressives- meaning something totally different than those who used the term to express their refusal to be a cog in the machine of the establishment Democratic system.
What are liberals? They are those who worked against labor until labor was too powerful, then they sought to co-op them. They are those who demeaned the anti-war activists of the 60s until the war was lost. They are those who laughed at LGBT folk until the entire country changed its mind. They are those who were afraid of Feminism until Second Wave Feminism was tamed and made part of the hegemony.
It has never been liberals who led on all the things liberals like to brag about: 40 hour work weeks, social security, medicare, civil rights. These things all happened because of leftists outside the system agitating and radicals in the streets, in the strikes, in the sit-ins, in the marches. Finally liberals fell in line and then took credit.