The measure, drafted by Representative Alexandria Ocasio-Cortez of New York and Senator Edward J. Markey of Massachusetts, was co-sponsored by Mr. Sanders as well as Ms. Gillibrand and Mr. Booker.

Education

One set of proposals Mr. Sanders presented in 2016 that drew widespread attention was the idea of making public colleges and universities tuition-free and significantly lowering student loan interest rates. He continued to highlight the cost of higher education in the months after the 2016 campaign.

A victim of his own success

As some of Mr. Sanders’s supporters have pointed out, the 2020 Democratic primary landscape looks far different than the one in 2016. This time, Mr. Sanders, an independent, will not be the only progressive opponent facing an establishment-backed front-runner. Instead, the 2020 Democratic primary field is already crowded with candidates, some who are newer to the national political scene than he is, and some who have embraced the very policies he championed in 2016.

[Check out the Democratic field with our candidate tracker.]

As The Times reported in December, Mr. Sanders is struggling to retain the support he garnered two years ago, when he was far less of a political star than he is today. His supporters have conceded that in some ways, Mr. Sanders is a victim of his own success.

“Ironically, Bernie’s agenda for working families will be the Democratic Party’s message in 2020, but he may not be the one leading the parade,” Bill Press, a talk show host who supported Mr. Sanders in 2016, said last year.

Recent turmoil

Mr. Sanders has also had a weak track record with black voters — a vital base in the Democratic Party — which could be a potential threat to his candidacy. On Sunday, The Times reported that interviews with nearly two dozen current and former advisers and staff members revealed an uneven commitment on the part of Mr. Sanders and his top advisers to organize and communicate effectively with black voters and leaders during his 2016 campaign.

And this year, Mr. Sanders is already dealing with another problem involving his 2016 staff: allegations from women who say they were mistreated or harassed during the campaign. Last month, after The Times published an investigation into complaints by female staff members, Mr. Sanders publicly apologized and later met with former staff members in an effort to calm the unrest.