After months of deliberation, Vermont Senator Bernie Sanders announced on Tuesday that he is running for president again in 2020. It will be Sanders' second consecutive bid for the Democratic nomination after losing to Hillary Clinton in 2016.

“I am asking you to join me today as part of an unprecedented and historic grassroots campaign that will begin with at least a million people from across the country,” he wrote in an email to supporters following an interview on Vermont Public Radio.

Sanders enters the 2020 race as one of the frontrunners – a remarkable turn for the democratic socialist who, three years ago, was viewed as a protest candidate from the political fringe. Today, Sanders is one of the most popular politicians among Democratic voters and his policy agenda – a suite of progressive proposals to expand health care, broaden the social safety net and make higher education free – has been embraced by many of the Democratic party's leading figures.

In the run-up to his announcement, Sanders and top aides insisted the decision would ultimately turn on a much simpler question: whether he was the best candidate to defeat President Donald Trump next year.

But in his Tuesday morning email and video announcing his run, Sanders – who described Trump as “a pathological liar, a fraud, a racist, a sexist, a xenophobe and someone who is undermining American democracy as he leads us in an authoritarian direction” — also set out loftier goals.

“Our campaign,” he said, “is about transforming our country and creating a government based on the principles of economic, social, racial and environmental justice.”

And in a message to rich and “powerful special interests,” Sanders warned: “They may have the money and the power. We have the people.”

By 7:30 a.m. ET, an aide told CNN, the campaign had received donations and sign-ups from all 50 states.

But Sanders’ increased influence has also invited stricter scrutiny from the political opponents, including a vocal faction inside the Democratic party who blame him for dampening support for Clinton ahead of her loss to Trump, the press, and even some of his most dedicated advocates.

The policy principles behind a second Sanders campaign are expected to be largely the same as in 2015 and 2016. Still, over the past few weeks he has begun to roll out or re-up proposals to combat economic inequality and fortify programs like Social Security. He is also planning to reintroduce his Medicare for all legislation in tandem with Washington Rep. Pramila Jayapal, who has taken over developing and shepherding the accompanying House bill.

Over two weeks in January and February, Sanders unveiled plans to buy more than 50 years of padding for Social Security by raising payroll taxes on income above $250,000 and hike the estate tax on the wealthiest Americans – suggesting a top rate of 77 percent on billionaire heirs.

“Our bill does what the American people want,” Sanders said in a statement ahead of the roll-out, “by substantially increasing the estate tax on the wealthiest families in this country and dramatically reducing wealth inequality. From a moral, economic, and political perspective our nation will not thrive when so few have so much and so many have so little.”