JUDY WOODRUFF: As the impeachment trial is set to start this next phase, we do not want to lose focus on the race for the White House. We are today exactly one year from the next president's inauguration and just two weeks from the Iowa caucuses. But, as Amna Nawaz reports, in South Carolina today, a show of unity among the Democratic contenders hides deeper divisions on the trail.
AMNA NAWAZ: A rare sight on the 2020 Democratic campaign trail, the presidential candidates all linking arms and shaking hands to honor the legacy of Dr. Martin Luther King Jr. After a weeklong feud about electability, progressive Senators Elizabeth Warren and Bernie Sanders appeared to reconcile, even as Sanders' critics said he dismissed sexism in politics.
QUESTION: Do you think that gender is still an obstacle for female politicians?
SEN. BERNIE SANDERS (I-VT), Presidential Candidate: The answer is yes. But I think everybody has their own sets of problems. I'm 78 years of age. That's a problem.
AMNA NAWAZ: Warren, when pressed by reporters, didn't take the bait.
SEN. ELIZABETH WARREN (D-MA), Presidential Candidate: I have no further comment on this. I have been friends with Bernie for a long time. We work together on many, many issues, and I have said all I'm going to say.
AMNA NAWAZ: The progressive cease-fire comes as a new fight brews, this time between Sanders and former Vice President Joe Biden.
QUESTION: What is your stance on Social Security?
AMNA NAWAZ: Biden on defense in Iowa after a deceptively edited video circulated on Twitter by a senior Sanders campaign adviser appeared to show Biden praising entitlement cuts.
JOSEPH BIDEN (D), Presidential Candidate: Paul Ryan was correct. When he did the tax code, what's the first thing he decided we had to go after? Social Security and Medicare.
AMNA NAWAZ: The full 2018 speech shows Biden was actually mocking Ryan, then the speaker of the House. On Saturday, Biden accused the Sanders campaign of misleading voters.
JOSEPH BIDEN: I have been a gigantic supporter of Social Security from the beginning. It's a flat lie. They have acknowledged that this is a doctored tape. And I think it's beneath and I'm looking for his campaign to come forward and disown it.
AMNA NAWAZ: But Sanders didn't back down from the attack.
SEN. BERNIE SANDERS: That's not true. And I think all of the media who has looked at it, it was not doctored. It was what it is.
AMNA NAWAZ: And suggested Biden was the one not being truthful.
SEN. BERNIE SANDERS: On the issue of Social Security, time and time again, Joe Biden has been clear in supporting cuts to Social Security.
AMNA NAWAZ: In this final push before the Iowa caucuses, other candidates urged Democrats to stay united. Former South Bend, Indiana, Mayor Pete Buttigieg said the focus should be on beating Donald Trump.
PETE BUTTIGIEG (D), Presidential Candidate: I know that the heat of competition will, I'm sure, continue to have all kinds of effects as we go into the final days before the caucuses and beyond, but, hopefully, we can also keep our eye on the bigger picture.
AMNA NAWAZ: Billionaire Tom Steyer agreed.
TOM STEYER (D), Presidential Candidate: This is going to be everyone getting together and making this happen together, or it is not going to happen.
AMNA NAWAZ: With just two weeks before the crucial first-in-the-nation contest, over half of Iowans remain undecided or open to changing their minds about a candidate. Even The New York Times editorial board was split on their endorsement. They backed both Warren and Minnesota Senator Amy Klobuchar on Sunday, declaring "May the best woman win". For the PBS NewsHour, I'm Amna Nawaz.