Democratic presidential candidates Bernie Sanders and Joe Biden picked up some key endorsements before tomorrow's primaries and caucuses. A total of 352 delegates are at stake Tuesday in six states, including the biggest prize, Michigan.

Senator Cory Booker, a former Democratic presidential candidate, announced his endorsement of Biden on Monday and joined "CBS This Morning" to discuss why.

"Well, look, it's a time for us to beat Donald Trump," Booker said. "And it became very clear to me that Joe Biden is the right person to do that. We have to unify and show our strength. And I think this Tuesday could be a pivotal day in our primary progress. But it's about time we start unifying as a party and begin the work to beat Donald Trump, and frankly, save our nation, humanity, address our common cause and our common challenges."

Booker's endorsement comes a day after another former rival, Senator Kamala Harris, announced her support for Biden, and Sanders won the backing of civil rights activist Jesse Jackson.

Co-host Tony Dokoupil asked Booker about endorsing someone he'd criticized on the campaign trail, including Biden's vote on the 1994 crime bill. "People cynical about politics, they see somebody like yourself criticize the guy, and then come out and back him. They say, what changed?"

"First of all, I love Joe Biden — he and I have had a great relationship for a long time. I wouldn't have run against him if I didn't think I was the best person to be the next president of the United States, but I'm out of the race. And as I look at him, I know factually from talking to him that some of my biggest issues — like criminal justice reform, like racial justice, like economic justice — he is going to be a strong leader on that, and can actually pull the country together, the kind of coalitions we need to actually make progress in those areas. I'm enthusiastic about this decision.

"I'm determined to beat Donald Trump," Booker said. "I know [Biden] is a leader, not just to beat Donald Trump, but to bring honor back to that office."

Co-host Michelle Miller asked Booker about the slate of Democratic presidential candidates, which began as a widely diverse field and is now led by two older white men. "What does it say about the Democratic Party?" she asked.

Booker replied, "I think a lot of people are waking up to the reality. African American voters in the South, African American voters here in Detroit, they have played a pivotal role in my entire lifetime in choosing a Democratic nominee. And what I'm excited about is to hear Joe Biden growing in enthusiasm, because he's always been talking about it, and understanding the issues of race and, frankly, racial reconciliation, racial justice, and even saying things now about choosing a black woman on the Supreme Court. I'm happy about the diversity of our nation. I'm happy to see the coalition that Joe Biden is building. I'm happy that I know confidently that he's going to govern in a way that brings us together as a nation, across racial, religious, even party lines, to begin to do the business. Because we know now more than ever our adversaries from China to Russia are building in strengthened determination to undermine our democracy. Our divisions as a country is our weakness. We need the candidate that can best unify all of us, and that is Joe Biden."

Co-host Anthony Mason asked Booker to address criticism by Bernie Sanders that the Democratic establishment was "ganging up" against Sanders, after both Senator Amy Klobuchar and former Mayor Pete Buttigieg withdrew from the race and endorsed Biden. Booker replied, "I don't think so. First of all, Bernie's my friend. I have a lot of respect for him and have worked with him in Washington, D.C. I just want us to get beyond pointing fingers at each other, trying to tear each other down. We can't tolerate that right now.

"The threat is in the White House. We're seeing that with the bungled way that [the president has] been handling this challenge with the coronavirus, to his denying of science. … This planet is in peril. We don't have time to fight against each other. We don't need to turn against each other; we need to turn to each other.

"And so I'm supporting Joe Biden because I think he's got the best chance of all the diversity in our party, of all the diversity across this nation, I think that in this particular time in history he's the person to unify us and help us to beat Donald Trump and lead forward together."