File picture: Bernie Sanders in Columbus, Ohio. Image Credit: AP

Senator Bernie Sanders, the Vermont independent and 2016 Democratic primary runner-up whose populist policy agenda has helped push the party to the left, announced Tuesday he was running for president again, embarking on a bid that will test whether he can retain the anti-establishment appeal he enjoyed with many liberal voters three years ago.

A self-styled democratic socialist whose calls for "Medicare for all," a $15 minimum wage and tuition-free public colleges have become pillars of the party's left wing, Sanders is among the best-known politicians to join an already crowded Democratic field and one of the most outspoken against President Donald Trump, whom he has repeatedly called a "pathological liar" and a "racist".

"Three years ago, during our 2016 campaign, when we brought forth our progressive agenda we were told that our ideas were 'radical' and 'extreme,'" Sanders said Tuesday in an early-morning email to supporters, citing those health, economic and education policies as well as combating climate change and raising taxes on wealthy Americans.

"Well, three years have come and gone. And, as result of millions of Americans standing up and fighting back, all of these policies and more are now supported by a majority of Americans," he said.

Sanders did not immediately announce where he would campaign first, nor did he disclose any staffing decisions for his new political operation.

A sensation in 2016, Sanders is facing a far different electoral landscape this time around. Unlike his last bid for the White House, when he was the only liberal challenger to an establishment-backed front-runner, he will be contending with a crowded and diverse field of other candidates, including popular Democrats like Sen. Elizabeth Warren of Massachusetts who have adopted his populist mantle.

While some presidential candidates have avoided direct broadsides against Trump, Sanders - ever combative - addressed his potential opponent head on.

"You know as well as I do that we are living in a pivotal and dangerous moment in American history," he said. "We are running against a president who is 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."