RNA - "What’s crazy is that we have a president who is a racist, a sexist, a xenophobe and a fraud," Sanders, who announced his 2020 candidacy, tweeted, The Hill reported.
"We are going to bring people together and not only defeat Trump but transform the economic and political life of this country," he added.
Trump, who occasionally derides Sanders as "Crazy Bernie", first weighed in Tuesday afternoon on the senator's decision to pursue the Democratic nomination.
"Personally I think he missed his time," Trump told reporters in the Oval Office, noting, "But I like Bernie because he is one person that, you know, on trade he sort of would agree on trade. I'm being very tough on trade, he was tough on trade."
Sanders is pursuing the Democratic nomination for the second time after placing second in the 2016 primary.
He joins a field of candidates including Sens. Elizabeth Warren (D-Mass.), Kamala Harris (D-Calif.), Amy Klobuchar (D-Minn.) and Cory Booker (D-N.J.).
Former Vice President Joe Biden and former Rep. Beto O'Rourke (D-Texas) are among those who are expected to announce soon whether they will run in 2020.