Appearing Friday on "New Day" on CNN, Sanders offered a caustic rejection of the Trump campaign's statement, put out by spokesman Jason Miller on Thursday night.
"Well isn't that something. My word! After eight years of having President Obama as president, Donald Trump now thinks he's a legitimate president. Well I'm just overwhelmed with emotion," Sanders told CNN's Chris Cuomo.
"Look, this is pathetic. And this goes to the root of what Trump's campaign is about," said Sanders. "Let's be clear -- it's about bigotry. You remember, let's all remember, that a few years ago, Donald Trump was the leader of the so-called 'birther' movement. And what the birther movement was about was not being critical of Obama. This is democracy, we can criticize Obama. It was delegitimizing the first African-American president."
"It is not acceptable for a candidate for president of the United States to be arguing whether or not our President was born in this country."
Sanders also drew a line between Trump's role in mainstreaming the birther movement in 2012 and his presidential campaign in 2016, saying that "I think this is part of his entire campaign of bigotry" and that Trump is "trying to appeal to those extreme, extreme, extreme extremes, who still believe that Obama was not born in America."
While he mounted a vigorous primary challenge to eventual Democratic nominee Hillary Clinton, Sanders pressed the case for a Clinton presidency Friday and sought to woo disaffected supporters of his who remain resistant her.
"Please, anybody who supports me -- do not think that Donald Trump in any way, shape or form reflects the point of view that I have," he said. Noting Trump's ability to dominate news cycles, Sanders said, "Don't worry about what CNN has on the air -- worry about your own lives, which candidate issue after issue after issue is better. And I think the answer is by far Hillary Clinton."
Sanders also presented the 2016 election as a stark choice, and accused Trump of lacking core beliefs.
"Either Hillary Clinton is going to become president or Donald Trump is going to become president. And I'm going to work as hard as I can to prevent Trump from becoming president," he said.
"Now, I ran against Clinton for a year. Of course my views are different than Clinton's on many issues. No question about it. The problem is Trump -- we don't know what he stands for. I can criticize Hillary Clinton for this, that, and the other thing. Trump literally changes his views every other day."