There've been heaps of stories about Democratic and Republican preparations for today's back-to-back Hill appearances by Robert Mueller (beginning 8:30 a.m.), which the networks are calling the most anticipated hearings in a decade.

• But guess who else has been prepping? Robert Mueller has been grilled in secret prep sessions, a source familiar with the process tells Axios.
• During the House Judiciary and Intelligence Committee sessions, Mueller will face questions about his 448-page report from 63 total lawmakers, with little overlap, Politico points out.

Yesterday afternoon, Axios' Jonathan Swan called Florida Republican Rep. Matt Gaetz to get a Mueller preview. Gaetz, along with Reps. Jim Jordan and John Ratcliffe, are expected to be the most aggressive Republican questioners.

• A source close to these three says they've spent weeks preparing to grill Mueller.
• "I can't talk right now," Gaetz told Swan. "I'm in Mr. Jordan's office working on my questions."

House Democrats spent 2½ hours in a "feisty" prep session — basically mock hearing — in the Rayburn House Office Building yesterday, NBC's Mike Memoli reported: "They say no one reads the book; everyone watches the movie."

• House Intelligence Chairman Adam Schiff told Axios' Alayna Treene that he wants to ask Mueller about the counterintelligence investigation that touched off the Russia/election scandal: "We want to find out the scope of what was investigated, what he didn't investigate. And what work is left to be done."

Go deeper: Search the Mueller report by groups, people and topics.

Negative stories about Trump administration immigration policies are getting much more online attention than stories that appeal to readers with immigration views aligned with the president, Axios' Neal Rothschild writes.

• That's based on data that social analytics company NewsWhip provided exclusively provided to Axios for our 2020 attention tracker.

Why it matters: The tracker — which measures reach and engagement of online posts— finds that immigration has been the campaign's top online issue by far.

• President Trump has continuously leaned into it because the issue is so popular with his base.
• But the stories that are getting the most traction are largely horror stories, and depictions of squalor at detainment facilities near the border.

By the numbers: Since March 1, among the top 100 articles about immigration, those likely to appeal to critics of the administration generated 23.1 million social interactions (likes, retweets shares). Those likely to appeal to readers who support the president's agenda have generated 11.1 million interactions.

• Among those top 100 articles, 50 appealed to administration critics; 33 had pro-administration sentiment; 17 had no clear leaning or no clear relation to the administration's immigration policies.

See past editions of our 2020 attention tracker here.

The Justice Department announcement yesterday that the Antitrust Division will review the practices of "market-leading online platforms" that could kick off a years-long probe, Axios' David McCabe writes.

• "The Department’s review will consider the widespread concerns that consumers, businesses, and entrepreneurs have expressed," DOJ said.
• Without naming the "market-leading" companies, DOJ said the probe will examine whether they "are engaging in practices that have reduced competition, stifled innovation, or otherwise harmed consumers."
• DOJ said it will look at "search, social media, and some retail services online" — wording that points to Google, Facebook and Amazon.

Why it matters: Antitrust action is one of the most significant steps a government can take to rein in a company.

• Such investigations can humble corporate giants and have major implications for the economy.

The big picture ... Tech suddenly faces a multi-front war in Washington:

• Support for more aggressive antitrust enforcement has grown on the left in recent years, culminating in Sen. Elizabeth Warren's proposal to break up Big Tech.
• President Trump has also been extremely critical of major tech companies over alleged — and unsubstantiated — censorship of conservative voices.
• The House Judiciary Committee is conducting its own investigation of major tech firms. Rep. David Cicilline (D-R.I.), chair of the antitrust subcommittee, said yesterday that Facebook, Amazon and Google hadn't adequately answered the panel's questions during a recent hearing.
• Lawmakers are also looking at ways to regulate major platforms over concerns about invasions of privacy, disinformation and criminal activity on the internet.

Between the lines: An antitrust prosecution would be a first for this generation of tech companies.

• The last major antitrust case in the sector was Justice's pursuit of Microsoft beginning in the late '90s.
• The case constrained Microsoft, giving Google the space to grow into a giant of its own.

🇬🇧 Happening today ... LONDON (AP) — Boris Johnson was due to enter 10 Downing St. as Britain's new prime minister on Wednesday, vowing to lead the U.K. out of the European Union and unite a country deeply divided over Brexit.

BBC senior North America reporter Anthony Zurcher, on how Americans see Johnson:

• "At this point in US history, it's hard for Americans to view any major political event outside the context of the rise and rule of Donald Trump — the crashing cacophony that drowns out all other thought."
• "Emmanuel Macron is elected president of France? A rebuke of Trumpism!"
• "The Liberal/National right-leaning coalition prevails in Australia? Trumpism triumphs!"

Trump himself got into the action, lauding Boris at the Turning Point USA Teen Student Action Summit: "He’s tough and he’s smart. ... They call him 'Britain Trump.' ... That’s what they need."

"Neil Armstrong’s Death, and a Stormy, Secret $6 Million Settlement," reports the New York Times' Scott Shane and Sarah Kliff:

• After the Apollo 11 hero's death in 2012 from complications following heart surgery at a Cincinnati hospital, his "sons contended that incompetent post-surgical care at Mercy Health — Fairfield Hospital had cost Mr. Armstrong his life."
• "The hospital defended the care, but paid the family $6 million to settle the matter privately and avoid devastating publicity, documents show."

Why it matters: The conflict "illustrates the controversial but common practice of confidential settlements in medical malpractice and other liability cases, which protect reputations but hinder public accountability."

• In addition, "it shows how the extraordinary renown of a figure like Mr. Armstrong can become a powerful hammer in negotiations."

Sen. John Kennedy (R-La.) bemoans runaway government spending, in a WashPost front-pager — "GOP shrugs at deficits, debt under Trump":

• "You don’t have to be Euclid to understand the math here. We’re like Thelma and Louise in that car headed toward the cliff."

But Kennedy told The Post he is open to supporting this week's agreement between President Trump and GOP lawmakers to lift the debt ceiling again, with hundreds of billions in new spending on top of it!

Snap's stock price shot up more than 6% yesterday after the company beat analyst expectations, posting more than $388 million in revenue, per Axios' Kia Kokalitcheva.

• Snap also added 13 million new daily active users this quarter.

Why it matters: Snap's share price has been steadily climbing since hitting its lowest price last December. The company is steadily seeking to make a comeback from its stumbles, like a mobile app redesign that failed to impress.

Facebook CEO Mark Zuckerberg "will have to personally certify that the company is taking steps to protect consumer privacy," under an FTC settlement expected to be announced today. (Wall Street Journal)

The Federal Reserve Bank of San Francisco will host what is believed to be the central bank's first research conference specifically on climate change, scoops Axios' Courtenay Brown.

• Why it matters: Climate change poses systemic risks to the soundness of the U.S. banking system, and the Fed is signaling its appetite to learn more. The conference comes at a time when the Fed is increasingly facing pressure to follow other central banks in considering the threats that global warming poses to the economy.

Airport lounges "are offering spa treatments and expansive views of the tarmac, and most are upgrading their food options. A few have even added playrooms for families traveling with children," writes the New York Times' Julie Weed.

• "A new lounge can cost more than $10 million ... and must be designed for flexibility, able to evolve over a decade as customers’ needs change," according to Alexander Dorow, United’s director of premium services.
• "The downside to the lounge improvements is that they can lead to capacity problems. Customers who might have gone straight to their gate now might ... go into American’s Admirals Clubs if they are offering free avocado toast that morning."