President Trump has publicly boasted that he could beat any of his 2020 Democratic challengers. Privately, several members of his campaign see a few who could pose a threat to his re-election, Axios' Alayna Treene reports.

• The campaign is in the early stages of building an attack strategy.

The bottom line: The three candidates who seem to concern the Trump campaign most are Joe Biden, Kamala Harris and Beto O’Rourke.

• That's in no particular order, and insiders all have different rankings.

Why they're worried:

Joe Biden: Several Trump advisers think the former VP is best positioned to take back the white Rust Belt voters Trump carried in 2016, making purple states like Michigan and Wisconsin more dangerous for the campaign.

• "A guy who loves his guns and God is not voting for Kamala Harris," a former Trump campaign staffer said. "But he would vote for Joe Biden. He’s a lot harder for them to demonize."
• "Biden is the only Democrat who passes the commander-in-chief test, which makes him appealing — particularly to swing voters," David Tamasi, former finance director of the Trump Victory Fund, told Axios.
• The other side: "He's low energy, and if he wins the nomination, all of the energy on the Dems' side will deflate like a balloon,” said a Republican operative close to the campaign.

Sen. Kamala Harris: Trump was impressed by Harris' massive crowd for her announcement event, according to White House aides.

• Some of Trump's advisers view Harris as a major threat because it's obvious to them that Trump hasn't figured out how to talk about her.
• He's given her no nickname and has yet to even test-drive a line of attack.
• A Trump adviser told Axios: "It's going [to be] hard for the president to attack her and debate her," because Democrats could cast attacks as racist and sexist.
• The other side: Several aides question whether she has enough experience and can sustain the momentum she'll need.

Beto O’Rourke: Former Arkansas Gov. Mike Huckabee told Fox Business last week that Republicans shouldn't underestimate O'Rourke. That view is shared by a number of people in Trump’s inner circle.

• "I have personally been very concerned about Beto for quite some time," a Trump campaign adviser said. "He seems to generate that liberal grassroots energy without being particularly in-your-face with his points of view. And he's charismatic."
• If O'Rourke were the nominee, he would also force the White House to spend a lot of money in Texas, which isn't the plan.
• The other side: Others close to Trump think the only reason O'Rourke was so successful in his Senate campaign is "because Sen. Ted Cruz ran a bad race," per the GOP operative.

The runners up: Several aides close to the president said Sen. Bernie Sanders shouldn’t be discounted as a formidable contender. "He is to the left what Trump was to the right in 2016,” Tamasi said.

• And while no one on the campaign thinks Elizabeth Warren could secure the Democratic nomination, some said she’s high on their radar because they see her as a great campaigner and her jabs often get under Trump's skin.
• "She can still do some damage," a former White House official said.

Statement from Kayleigh McEnany, press secretary for the Trump campaign:

• "It doesn’t matter who emerges from the Democrat convention in 2020, because that candidate will be beat up, low on funds, without a national operation, and saddled with the socialist policy positions demanded by the extreme left of their party."

Here's something that ought to catch Democrats' attention: Swing voters at a recent focus group in Wisconsin hadn't heard of either the Green New Deal or "Medicare for all," Axios' Alexi McCammond reports.

• Why it matters: These are Democrats' biggest policy staples heading in the 2020 presidential election. They're talking about them all the time, and the ideas are even being weaponized by the right to label the entire Democratic Party as socialists. But none of that is breaking through in this key battleground state.
• This takeaway comes from the Engagious/FPG focus group of swing voters we watched earlier this month, which included eight Barack Obama/Donald Trump voters and four Mitt Romney/Hillary Clinton voters.

Details: Not a single person had heard of — or could explain — the Green New Deal.

• "I would only say that it's an environmental policy because it has the word 'green' in it," said one woman.
• Nine of the 12 people said they'd heard almost no news at all about "Medicare for all" in the past several months.

Half of the participants had never even heard the phrase "Medicare for all" until they walked in the room that night in Appleton, Wisconsin.

• "I think Bernie Sanders has said something about it," said Carla N., a 53-year-old Romney/Clinton voter.
• A majority of these swing voters have no idea which party is pushing the plan.
• Because of that, they don't view either the Green New Deal or "Medicare for all" favorably or unfavorably.

The bottom line: Democrats might have an opportunity to capitalize on Trump fatigue in 2020 in this critical battleground state — but they're a long way from winning over swing voters on the ideas that will define their campaigns.

For your Tuesday radar: The 2nd Circuit Court of Appeals in New York will hear a case against Trump for blocking critics on Twitter, Axios' Sara Fischer reports. 

• Why it matters: In January, the Virginia-based 4th Circuit Court of Appeals ruled that government officials can't block constituents on social media accounts that they use for official business, based on its interpretation of the First Amendment. If the 2nd Circuit interprets the law differently, it's likely that the issue could find its way to the Supreme Court.

Yes, but: If the 2nd Circuit agrees with the 4th Circuit's interpretation of the law, it would set an important precedent on how public officials, especially the president, can use their official social media accounts.

• It would mean that government officials' social media accounts should be treated the same way as access to physical public forums like town halls, where no one can be blocked from participating based on the content of their speech.

What they're saying: 

• The White House has argued that the @realdonaldtrump Twitter handle, which was created before the president ran for office, is not his official @POTUS45 government handle, and thus the president has the right to block whoever he wants. 
• The plaintiffs — i.e., those who have been blocked — argue that the White House has used the @realdonaldtrump Twitter handle for official communications and thus he shouldn't be able to block people from the account based on the views they express.

Be smart: Legal experts point to examples in the past where White House officials have declared statements from the @realdonaldtrump handle to be official statements from the president.

• "I don’t think the government’s argument will hold up in court," said Joshua Geltzer, executive director of the Institute for Constitutional Advocacy and Protection at Georgetown Law Center.
• "The courts have been rejecting this type of argument."

The bottom line: Trump's prolific use of social media use is testing the legal limits of the First Amendment. 

The Supreme Court this week will wade back into a fundamental question about American democracy: whether partisan gerrymandering can ever go too far, Axios' Sam Baker reports.

The big picture: State lawmakers have gotten a lot more sophisticated and a lot more aggressive about redrawing their state’s legislative districts to help their party stay in power.

• "I think electing Republicans is better than electing Democrats. So I drew this map to help foster what I think is better for the country," the architect of North Carolina’s 2016 redistricting process said. His plan is now before the high court.

Driving the news: The justices will hear two hours of oral arguments Tuesday: one hour about North Carolina’s map and one hour about a Democratic-led gerrymander in Maryland. Rulings are expected in June.

Why it matters: Critics say extreme partisan gerrymandering undermines the basic premise that each person’s vote counts equally.

• In North Carolina, for example, Republicans won 53% of the popular vote in 2016, yet ended up controlling 77% of the seats in the state legislature — the exact breakdown they were aiming for when they drew their map.

The other side: The most interesting debate here isn't partisan, but rather a divide between voting-rights advocates and conservatives who argue that redistricting is a quintessentially political process and the courts should stay out.

Where it stands: The Supreme Court has never struck down a partisan gerrymander. It has never said a state legislature crossed the line in trying to secure a partisan advantage — in fact, it has never even said whether there’s a line to cross.

• Then-Justice Anthony Kennedy seemed poised to draw such a line during the court's last term, but ultimately punted ahead of his retirement.
• Chief Justice John Roberts, who has become the court’s ideological center in Kennedy’s absence, seemed concerned last time around about wading into a political process.

The House will consider the Paycheck Fairness Act this week. Members will also vote on Rep. Joe Kennedy’s resolution condemning the Trump administration’s ban on transgender individuals serving in the military.

• Tuesday: Speaker Nancy Pelosi speaks at AIPAC. She'll also host a House bipartisan leadership meeting with Israeli Prime Minister Benjamin Netanyahu.
• Also on Tuesday, the House will vote to override the president’s veto of the disapproval resolution on his emergency declaration. House Democrats will also unveil their legislation aimed at protecting people with pre-existing conditions and lowering health costs.
• Wednesday: The House Intelligence Committee will hold a hearing with Russian-born real estate developer Felix Sater, who is expected to discuss his work with Michael Cohen on plans for a Trump Tower in Moscow.
• Throughout the week, several members of the Trump administration will testify about the president's budget proposal before a House Appropriations subcommittee.

The Senate will have a cloture vote on the motion to proceed on the Green New Deal.

• Monday: The Senate will have a cloture vote on Arizona Judge Bridget Bade's nomination to the U.S. Court of Appeals for the 9th Circuit.
• The Senate will also have a cloture vote on the motion to proceed to the disaster supplemental appropriations package.

President Trump's schedule, per a White House official:

• Monday: Trump holds a bilateral meeting with Netanyahu. He'll also host the 2018 Stanley Cup Champions, the Washington Capitals.
• Tuesday: Trump meets with members of Congress to discuss trade. He'll also participate in a working dinner with Netanyahu.
• Wednesday: Trump has lunch with Vice President Pence. He'll later present the Medal of Honor to the family of Travis Atkins, an Army staff sergeant killed in Iraq in 2007.
• Thursday: Trump delivers remarks at a Make America Great Again rally in Grand Rapids, Michigan.