JUDY WOODRUFF: With just three weeks until Iowa's first-in-the-nation presidential caucuses, an increasingly close race there is exposing new rifts among the candidates. Lisa Desjardins is back with this report.
SEN. BERNIE SANDERS (I-VT), Presidential Candidate: When millions stand up and fight back, there is nothing that will stop us.
LISA DESJARDINS: The Democratic presidential field, still numbering in the double digits, narrowed today by one.
SEN. CORY BOOKER (D-NJ), Presidential Candidate: Today, I am suspending my campaign for president.
LISA DESJARDINS: New Jersey Senator Cory Booker ended a campaign that had pushed for national unity, just as Vermont Senator Bernie Sanders and Massachusetts Senator Elizabeth Warren abandoned their year-long nonaggression pact.
SEN. ELIZABETH WARREN (D-MA), Presidential Candidate: I was disappointed to hear that Bernie is sending his volunteers out to trash me. I hope Bernie reconsiders and turns his campaign in a different direction.
LISA DESJARDINS: That was Warren responding to a leaked Sanders campaign script distributed to volunteers and published by Politico. It attacks Warren by charging that she appeals only to highly educated, more affluent voters. And it says that she's bringing no new bases into the Democratic Party.
SEN. BERNIE SANDERS: Presidential Candidate: 广东快乐十分Elizabeth Warren is a very good friend of mine.
LISA DESJARDINS: Sanders denied a role in the role in the new offensive strategy and distanced himself from the campaign volunteers involved.
SEN. BERNIE SANDERS: People sometimes say things that they shouldn't. You have heard me give many speeches. Have I ever said one negative word about Elizabeth Warren?
LISA DESJARDINS: The tension comes as new polling shows a razor-thin four-way fight in Iowa. Friday's Des Moines Register/CNN poll had Sanders narrowly in the lead, earning 20 percent support from likely Democratic caucus-goers, with Warren statistically tied with him. Following close behind, the former mayor of South Bend, Indiana, Pete Buttigieg, and former Vice President Joe Biden at 15. Biden topped today's Monmouth University poll with 24 percent, with Sanders, Warren and Buttigieg tied in a close second.
PETE BUTTIGIEG (D), Presidential Candidate: Thank you!
LISA DESJARDINS: Buttigieg also spent the past few days in Iowa, with a Des Moines touting the endorsement of a prominent Iowa congressman. But the candidate also faced some controversy.
PETE BUTTIGIEG: I think your facts are a little wrong, so I'd love a chance to talk with you about it.
LISA DESJARDINS: The former mayor confronted protesters in the crowd challenging his record on race, including on policing and housing for African-American communities.
PETE BUTTIGIEG: Can we agree that we can talk about this respectfully?
WOMAN: We have been trying to talk to you since South Bend, Pete.
LISA DESJARDINS: With a number of candidates camping out in Iowa, others are putting their marks on the states that follow.
ANDREW YANG (D), Presidential Candidate: Hello, Dover, New Hampshire! How are you?
LISA DESJARDINS: Entrepreneur Andrew Yang hoping for a surge in New Hampshire.
ANDREW YANG: Screw Iowa. No, I'm kidding.
ANDREW YANG: Kidding!
LISA DESJARDINS: Biden looking to cement his lead in Nevada and court Hispanic voters there.
JOSEPH BIDEN (D), Presidential Candidate: Now, I can assure you of one thing. My Cabinet, if I'm elected, and my administration will look like America. It will look like America.
LISA DESJARDINS: And former New York City Mayor Michael Bloomberg campaigning with television personality Judge Judy in Texas. Bloomberg has staked his nomination chances on the Lone Star State and others which vote in March.
MICHAEL BLOOMBERG, Presidential Candidate: Judge, I am so honored to have you here. I'd love to tell you that I watch you all day every day, but I have a few other things to do.
LISA DESJARDINS: A dozen candidates still crisscrossing the country, but just six of them will face off tomorrow night in Des Moines in the seventh Democratic presidential debate.
For the PBS NewsHour, I'm Lisa Desjardins.