That national organization built over the last four years and maintained by Sanders and his allies is more robust than anything any other candidates in the race, including Joe Biden, who's set to announce Thursday.
Sanders is likely to raise the most money of anyone in the field. He raised $18+ million in the first three months of 2019, with 84% of those contributions coming in at under $200. That was the biggest total of any 2020 Democrat. There's every reason to think he can keep it up; he raised $237 million for his 2016 race against Clinton.
His path to the nomination is the easiest to see, with Iowa's caucuses dominated by liberals and his geographic proximity to New Hampshire.
Sanders' liberalism -- once considered radical -- is now very much en vogue within the party. He's been in that space for a very long time.