In a video to supporters, he said, "Our campaign is about transforming our country and creating a government based on the principles of economic, social, racial, and environmental justice".
The progressive populist mounted a fierce challenge to front-runner Hillary Clinton in the 2016 election. He opens his campaign with a nationwide organization and a proven small-dollar fundraising effort.
Sen. Bernie Sanders (I-Vt.) has some pretty ambitious ideas about tackling rising health care costs, a warming climate and the stagnant minimum wage.
Mr Sanders is the longest-serving independent in congressional history, but competes for the Democratic nomination as he says standing as a third-party candidate would diminish his chances of winning the presidency.
"Three years have come and gone". He told CBS "these ideas and many more are now part of the political mainstream".
"If past year we did not have the filibuster, the Trump administration and the GOP majority could have rammed through an incredible range of laws that those same progressive groups would find incredibly destructive", Sen.
Four years later, Mr Sanders' policies remain the same but much has changed on the political landscape.
Opec tightens its taps as global oil demand stalls
The group made a strong start to the strategy, slashing output by 930,000 bopd to the lowest in nearly four years, the IEA said. Hassan Rouhani's government would face even a much worse scenario if the US decides to bring Iran's oil exports down to zero.
If the Democratic party is going to do well in the future, I think they have to reach out to those independents - including, by the way, a lot of young people and a lot of people of color - and bring them into the Democratic party. Elizabeth Warren of MA, who will likely be his chief rival in the race. Even if the Democrats pass him over for the nomination again, he's already done more to shape the outcome than anybody else in the field of declared candidates right now.
Mr Sanders, 77, chose to make his announcement in a rather low key manner, making the declaration in an email sent to supporters.
"There are a lot of people I know personally who work hard for a living, who make $40,000, $50,000 a year who know a lot more about politics than, in all due respect, does [Howard] Schultz", said Sanders. Elizabeth Warren, Kamala Harris, Amy Klobuchar and Cory Booker. The field could also grow, with a number of high-profile Democrats still considering presidential bids, including former vice president Joe Biden and former Texas Rep. Beto O'Rourke.
Donald Trump himself has expressed his admiration for Sanders, saying he would have preferred to run against him than Hillary in 2016. He's using the same issues from his 2016 campaign which resonated with younger voters.
Last month, he joined Booker at an event in Columbia, South Carolina, marking the Martin Luther King Jr. holiday.
After coming a few hundred delegates short of victory in 2016, Sanders begins a 2020 race with some advantages.
Other Democratic presidential hopefuls also share concerns about going nuclear to kill the filibuster, including Sens.
In January, he was forced to publicly confront allegations of sexual harassment by staffers on his 2016 campaign.
Already the house organ of the Democrat Party, the New York Times, is fretting.