Sen. Cory Booker was making critical campaign stops this weekend in Iowa and South Carolina, as the crowd of 2020 Democratic presidential contenders around him continued to expand.

"This campaign can't be about tearing someone down," Booker said as he campaigned in the early primary state of South Carolina. "It's about restoring our civic grace and appealing to our better angels."

Sen. Elizabeth Warren (D-Massachusetts) announced her run for the Democratic nomination Saturday in the former mill town of Lawrence, Massachusetts. Warren, who graduated from Rutgers Law School in Newark and lived in North Jersey for a time, currently leads Booker in the polls in the first-in-the-nation Iowa caucus.

On Friday and Saturday, Booker made his first trip to Iowa. He's hoping his positivity and what he and supporters say are his record of working across the aisle can help him stand out in the Democratic field.

Booker will remain in South Carolina through Monday, and he's due to hit New Hampshire later this month.

Booker and the contenders were joined by another 2020  hopeful on Sunday as Sen. Amy Klobachur (D-Minnesota) announced she too is running for president.

Former New York City Mayor Michael Bloomberg says he'll decide whether to run by the end of this month. Vermont Sen. Bernie Sanders, former Vice President Joe Biden and former Texas Rep. Beto O'Rourke are also said to be thinking about joining the race.