In 2016, establishment Democrats saw Bernie as a one-off reaction from Democrat voters, especially young voters, offended by Hillary's deep establishment ties, her profitable dances with corporations and totalitarian nations, and her unpleasant personality.  They were convinced that in 2020, if Bernie were placed against a more attractive, compelling, younger, and less obviously corrupt Democrat field, he'd quickly vanish.  They were wrong.

As Vice explains:

Sen. Bernie Sanders closed out his 2019 comeback story with a field-beating fundraising haul, adding more momentum to his candidacy just one month before the first votes are cast in the Democratic presidential primary.

The Sanders campaign announced on Thursday morning that the Vermont Senator raised $34.5 million in the last quarter of 2019 from more than 1.8 million donations. That brings the total number of donations Sanders has received so far for his campaign to over 5 million, more than any other campaign.

December was also his best month yet, with $18 million taken in from over 900,000 donations — 40,000 on the final day of the quarter, alone.

Considering that several Democrats are still fighting for money from the same cohort of voters, that's a staggering haul.  Of course, it doesn't match what President Trump has collected, but Trump does not have to fight with fellow Republicans for the money.

All of this suggests that if Bernie is not blocked by the machinations that Hillary employed in 2016, this 78-year-old heart attack survivor has a good chance of becoming the Democrat Party's presidential candidate — and sensible Democrats are terrified.  Ronald Radosh explains why a Bernie candidacy would be a nightmare for the Democrats:

Should Sanders actually pull off the feat of capturing the nomination, Donald Trump would have been given a gift that almost assures his re-election. Trump already refers to the Democrats as "the far-left Democrats" and has branded all of the potential candidates as socialists. "We will not live in a socialist America," he said to cheers in one of his rallies, suggesting that such an outcome would occur should any Democrat win the White House. With Sanders as the presidential candidate, he could say without distortion that the Vermont senator's end goal is a socialist United States.

[snip]

[A] Sanders nomination would put many states in play that Democrats had easily won for a quarter-century. Sanders is where he is today in part because no one has really attacked him. But just wait until Republicans spend a billion dollars painting him as an extremist. He'd win Vermont, Massachusetts, Rhode Island, Maryland, Delaware, California, Washington, and Hawaii, and also probably New York and Illinois. But a huge number of usually-blue states would be up for grabs. He would also find that Democratic candidates would run away from him. Many candidates running for governor, the Senate, and the House in purple states and districts would refuse to campaign with him, or at best make a half-hearted quick appearance.

In 1972, the candidate hated most by the left and the liberals, Richard Nixon, became President of the United States. Is that what today's Trump opponents really want to repeat?

Radosh also calls out Alexandria Ocasio-Cortez, whom Bernie has had glued to his side of late.  Radosh rightly attacks the gross historic ignorance that sees her castigate as a "fascist" anyone who disagrees with her extreme politics:

What Ocasio-Cortez has really done is both to reveal her ignorance and to repeat the favorite accusation of members of the Old Left (Communists and their allies in the 40s and 50s) when they proclaimed all of their enemies to be fascists. They did it so many times that if a real fascist came along and was close to taking power in our country, no one would listen to their accusation. So prevalent was the accusation that the American Communist Party publicly called President Harry S. Truman — a president who desegregated the armed forces, favored universal health care coverage, and vetoed the repressive McCarran Internal Security Act — a fascist.

Radosh has more to say, and it's worth reading.