Sanders first entered Congress in 1991. In 1991 Bill Clinton was the National Chairperson of the centrist Democratic Leadership Council, and one year later Clinton was elected president . Four years later, in 1996 Clinton proclaimed that "the era of big government is over"during his State of the Union address.
Sanders saw his personal popularity inside Vermont increase consistently but, prior to Howard Dean, no politician from tiny Vermont gained much national respect, certainly not a supporter of Jesse Jackson's "Rainbow Coalition" when being positioned as a "New Democrat" became all the rage. He was ideologically poorly positioned to make alliances inside the then prevailing national Democratic Party, even if he wanted to. His was not the message that the mainstream Democratic Party was then eager top embrace. Prior to stepping forward to run for President in 2016, due in large part to his politics Sanders had no major financial backers and no ready access to national media.
It is relevant to note that it was because of Bernie Sanders that Community Health Centers became a key component of the Affordable Care Act, and the role that they are playing now is crucial in the fight we are waging against Covid 19. I have no problem giving credit to John Conyers for his Medicare For All bill dating back to 2003, but of course Conyers too was not able to move it forward to become law. All great progressive movements have antecedents to acknowledge. I an happy to acknowledge Eugene Debbs and Norman Thomas also. Great credit obviously is due to FDR and LBJ and to Organized Labor and to MLK Jr's Poor People's Campaign. Martin Luther King Jr. didn't pass legislation, rather he advocated for and organized around grand ideas. I believe Bernie's greatest contribution has been as an advocate for grand ideas that he has helped build a movement for.
Progressive ideas have existed since before the American Revolution. Thomas Paine is a personal hero of mine. I don't care when and where they originate though I always honor those who push for them against the prevailing political grain. Getting progressive legislation passed into laws that matter is critical, but so is raising the public profile of such issues so that a constituency forms to fight for the passage of that legislation. Bernie Sanders did not "invent" all of the planks from his 2016 presidential campaign, but he promoted them effectively on a national stage and now much of what he ran on as an outsider then is considered to be mainstream Democratic Party policy dogma, or at least no longer seen as unrealistic to contemplate working toward.