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WASHINGTON — Bernie Sanders is now in the lead in Iowa — albeit within the margin of error.

And what does he do with that lead three weeks before the first Democratic nominating contest? He and his supporters are on the attack.

It’s Bernie vs. the World, as NBC’s Gary Grumbach puts it.

On Saturday night — a day after the Des Moines Register poll showed Sanders narrowly leading the Dem race — his campaign blasted Joe Biden’s vote to authorize the Iraq war.

“It is appalling that after 18 years Joe Biden still refuses to admit he was dead wrong on the Iraq War, the worst foreign policy blunder in modern American history,” said senior adviser Jeff Weaver in a statement from the campaign.

On Sunday morning, Nina Turner, the national co-chair of Sanders’ campaign, went after Biden on the issue of race in South Carolina.

“Will our community side with former Vice President Joe Biden, who has repeatedly betrayed black voters to side with Republican lawmakers and undermine our progress?” Turner wrote in a published op-ed in South Carolina’s The State newspaper.

And then came the Politico report of a volunteer script from the Sanders campaign that knocks Elizabeth Warren and her support.

“The script instructs Sanders volunteers to tell voters leaning toward the Massachusetts senator that the ‘people who support her are highly-educated, more affluent people who are going to show up and vote Democratic no matter what’ and that ‘she's bringing no new bases into the Democratic Party,’” Politico writes.

“I was disappointed to hear that Bernie is sending his volunteers out to trash me,” Warren told NBC’s Ali Vitali on Sunday afternoon. “Democrats want to win in 2020 we all saw the impact of the factualism in 2016 and we can't have a repeat of that.”

Sanders responded afterward, per NBC’s Grumbach: “We have over 500 people on our campaign. People do certain things, I'm sure that in Elizabeth's campaign, people do certain things as well. But you heard me for months, I have never said a negative word about Elizabeth Warren, who is a friend of mine. We have differences on issues, that's what campaigning is about. But no one is gonna be attacking Elizabeth.”

If this all sounds familiar, it’s similar to what we saw in 2016.

Sanders, his supporters and his surrogates — like Weaver and Turner — go on the attack; Sanders downplays or dismisses the attacks; and the party becomes more divided.

And the memories of 2016 remain Sanders’ top challenge in winning the Dem nomination and uniting the party in 2020.

“Doesn’t surprise me about Bernie,” Iowa state Sen. Claire Celsi, a top Warren supporter, told the New York Times. “He went straight to the gutter with Hillary. More of the same.”

Data Download: The number of the day is … 58 percent

58 percent.

That’s the share of Iowa Democratic caucus-goers who say they could be persuaded to support a different candidate (45 percent) or say they haven’t decided on a first choice yet (13 percent), according to the latest Des Moines Register/CNN Iowa poll.

Four-in-ten said their minds are made up, up 10 percent from November.

Trump’s credibility deficit on the Soleimani killing

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Key Trump national security officials went on the Sunday shows, including “Meet the Press,” to argue that the killing of Iranian Gen. Qassem Soleimani was based on an “imminent” security threat.

The problem for the Trump administration, however, is that its message has been all over the place – on top of the president’s already-established credibility deficit.

Here was the president on Friday night: “I believe it would have been four embassies [under attack]. And I think that, probably, Baghdad already started. Baghdad certainly would have been the lead. But I think it would've been four embassies. Could've been military bases. Could've been a lot of other things, too.”

But here was the president’s Defense secretary on CBS yesterday: “Defense Secretary Mark Esper said Sunday that he ‘didn't see’ specific evidence that Iran was readying to attack four U.S. embassies, as President Donald Trump claimed last week, though Esper said he shared Trump's view that such an attack was ‘probably’ in the works,” per NBC News.

“‘What the president said was he believed that it probably and could've been attacks against additional embassies,’ Esper told CBS.”

And now comes this report from NBC’s Carol E. Lee and Courtney Kube: “President Donald Trump authorized the killing of Iranian Maj. Gen. Qassem Soleimani seven months ago if Iran's increased aggression resulted in the death of an American, according to five current and former senior administration officials.”

Seven months ago.

Tweet of the day

Narrator: it’s 53 degrees in Washington DC right now. https://t.co/gFH3HOqYUK — Molly Jong-Fast (@MollyJongFast) January 13, 2020

2020 Vision: Buttigieg gets endorsement from Iowa congressman

Rep. Dave Loebsack, D-Iowa, endorsed Pete Buttigieg on Sunday, becoming the second member of Iowa’s congressional delegation to pick a side in the Dem 2020 race, Politico writes.

Earlier this month, Rep. Abby Finkenauer endorsed Joe Biden.

And at publication time this morning, the Biden campaign announced that Rep. Colin Allred, D-Texas, endorsed the former vice president.

On the campaign trail today

It’s a relatively quiet day with tomorrow’s Dem debate in Iowa: Pete Buttigieg holds town halls in Winterset and Ames… Jill Biden stumps for her husband in the Hawkeye State… And Andrew Yang, Tom Steyer and John Delaney also campaign in Iowa.

Dispatches from NBC’s campaign embeds

At his first Iowa town hall of 2020 yesterday, Pete Buttigieg was interrupted by protesters from the South Bend Black Lives Matter group, per NBC’s Priscilla Thompson.

“Around ten minutes into his speech someone near the stage called for medical support (this was meant to be an attention grabber it seems), at this point a protester approached the stage and asked Buttigieg a question [that] was inaudible to those not in the immediate area. Buttigieg and the protestors engaged in a brief back and forth, which included a question about Buttigieg’s signature ‘1,000 Houses in 1,000 Days’ program as mayor, which some say disproportionately negatively impacted homeowners of color. Buttigieg told the protester he had his facts wrong.”

“Soon ‘Black Lives Matter,’ chants broke out and were drowned out by ‘Boot Edge Edge.’ Eventually the protesters were escorted out shouting ‘Black Lives Matter,’ and ‘Eric Logan’ along the way.

The Lid: Steyer’s Millions

Don’t miss the pod from Friday, when we looked at how Tom Steyer’s ad spending in early states like Nevada and South Carolina helped him qualify for Tuesday’s Democratic debate.

ICYMI: News clips you shouldn’t miss

Trump authorized the killing of Iranian Maj. Gen. Qassem Soleimani seven months ago if Iran's increased aggression resulted in the death of an American, five current and former senior administration officials tell NBC News.

Nancy Pelosi says she doesn’t regret holding on to the articles of impeachment.

Pete Williams looks at what Chief Justice John Roberts’ role would be in the impeachment trial.

A key New Hampshire union is backing Sanders.

The Trump administration is scaling back how much federal workers can telework.

Mitch McConnell is crafting his impeachment strategy with his own re-election race in mind.

2020: Trump eyes Sanders

Trump is starting to focus more and more on Bernie Sanders.

Michael Bennet says that an impeachment trial will be disruptive to the presidential campaign.

POLITICO reports on how Sanders is consolidating progressive support.