In the homestretch before Monday's Iowa caucuses, Bernie Sanders overtook Joe Biden as the Democratic candidate getting the most attention online, Neal Rothschild writes using NewsWhip data provided exclusively to Axios.

• Why it matters: Biden had led for the 17 previous weeks. The numbers show that at the most important moment of the contest so far, Sanders' profile is peaking, accompanied by climbing poll numbers.
• Online engagement reflects grassroots enthusiasm that often isn't fully captured by polling.

Between the lines: Most positive online Biden sentiment has been around stories that point to his ability to beat President Trump.

• Sanders supporters are looking to make the same case. In January, the biggest single story around him wasn't the dustup with Elizabeth Warren or being slammed by Hillary Clinton, but a poll showing that Sanders leads Trump by the biggest margin of any Democratic candidate.

By the numbers: Stories about Sanders have generated more interactions on social media (likes, comments, shares) than stories about Biden over each of the past three weeks, totaling 20.1 million interactions to Biden's 14.2 million.

• When Biden held the top spot, part of his lead could be attributed to his role in the saga that led to Trump's impeachment.
• During that period, Sanders has eaten into Biden's national polling lead, narrowing a 10-point deficit to 5 percentage points, per the RealClearPolitics average. And he has taken a slight lead in Iowa, per FiveThirtyEight.

Online interest hasn't pushed Sanders past Biden in TV coverage. On cable news this month, Biden has been mentioned 9,300 times to Sanders' 7,400, according to the Internet Archive Television News Archive.

• Sanders edges on network evening news, with 195 mentions to Biden's 157.

Go deeper: Past installments of the 2020 Attention Tracker.

President Trump is scheduled to deliver his State of the Union address on Tuesday night, sandwiched between Democrats' Iowa caucuses on Monday night and a final vote at the impeachment trial on Wednesday.

• Senate Democratic Leader Chuck Schumer's last-ditch effort to get witnesses — forcing amendments to subpoena John Bolton, Mick Mulvaney and others — was shot down, Alayna Treene reports from the chamber.

💣 Another Bolton bomb: The N.Y. Times reported that former national security adviser John Bolton says in his forthcoming book that Trump asked him in May to be sure the incoming Ukrainian president met with Rudy Giuliani.

• Rudy was planning a trip to Ukraine to discuss investigations Trump wanted.

🥊 Romney shunned: Matt Schlapp, chairman of the American Conservative Union, disinvited Sen. Mitt Romney (R-Utah) from this year's CPAC after he voted against witnesses.

• 🇺🇦 Marie Yovanovitch, former U.S. ambassador to Ukraine, retired and yesterday was her last day as a State Department employee. (WSJ)

The vote against witnesses:

How it's playing:

• Go deeper: The trial, day by day.

With the U.S. declaring a public health emergency, President Trump signed an order barring entry to most foreign nationals who have visited China within the last 14 days, the coronavirus's longest incubation period, AP reports.

• China slammed the U.S. controls.
• Why it matters, per the WashPost: The measures mark a dramatic escalation of the administration's response "after initially downplaying potential risks."

Most cases so far have been people who visited China or their family members, but fears are rising of human-to-human transmission in other countries that may not be prepared.

• The virus's rapid spread in two months prompted the World Health Organization to declare it a global emergency on Thursday.

Delta and American Airlines yesterday suspended all flights between the U.S. and China.

• Why it matters: "The halt of flights connecting the U.S. with Beijing and Shanghai underscores the rising concern, since both cities are hundreds of miles from the focus of the outbreak." (Bloomberg)

This image, from video by Jenna Fabian, shows an L.A. Fire Department firefighter on a ladder rescuing a man who had climbed out on the side of a 25-story high-rise apartment, escaping flames from a burning balcony.

• 13 people were hurt in the fire at the Wilshire Boulevard tower — built nearly 60 years ago, and exempt from internal-sprinkler requirements under a loophole that City Council members are now trying to close. (L.A. Times)

The Lakers honored Kobe and Gigi Bryant by marking their courtside seats with their jerseys.

• Portland Trail Blazers won, 127-119.

And outside Staples Center ...

Michael Beckerman, founding president of the Internet Association, is stepping down to run TikTok's D.C. policy shop, Axios' Margaret Harding McGill scooped.

• Why it matters: Beckerman is leaving the trade group at a time of industry turbulence to join a video-sharing app that's facing Washington headwinds.
• TikTok faces criticism from lawmakers over issues ranging from censorship to children's privacy.

Beckerman will join TikTok as vice president of U.S. public policy in March.

• He leaves IA after 8 years, having helped start the organization with 10 members in 2012. IA now has more than 40 members, including Google, Facebook and Amazon, and six offices working on federal, state and international policy.

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An image of the clock face of Big Ben was projected on 10 Downing Street, residence of British Prime Minister Boris Johnson as Britain officially left the European Union at 11 p.m. London time.

• "Brexit parties were held in some pubs and social clubs as well as in London's Parliament Square [below], as the country counted down." (BBC)
• Why it matters: "In its biggest shift since losing its global empire, [the UK turns] its back after 47 years on the post-World War Two project that sought to build the ruined nations of Europe into a global power." (Reuters)

Ads to watch for during tomorrow's Super Bowl (clockwise from upper left), narrated by AP's Mae Anderson:

• Doritos: Lil Nas X, whose “Old Town Road,” was the smash hit of the summer, has a dance-off with grizzled character actor Sam Elliott (left in photo). Billy Cyrus, featured in a remix, makes a cameo.
• Amazon: Ellen DeGeneres (left) and her wife, actress Portia de Rossi (right), imagine life before Alexa. A maid named Alexa throws burning firewood out a window when she's asked to turn the temperature down; an Old West pioneer asks his companion Al to “play that song I like.” In one of the night's few political allusions, a newsboy says his newspaper has “fake news,” and a character resembling Richard Nixon asks his secretary to remind him to delete his tapes. She tells the camera she won’t do it.
• Michelob Ultra: Wrestler-turned-actor John Cena (right in photo) tries to convince "Tonight Show" host Jimmy Fallon (left) that there’s a “lighter side” to working out. Cameos: The Roots and sprinter Usain Bolt.
• Cheetos: A man with bright orange dust on his hands keeps moving furniture and performing office tasks. MC Hammer — “Hammer pants” and all — keeps popping up to say: “U Can’t Touch This.”