President Trump has publicly boasted that he could beat any of his 2020 Democratic challengers. But privately, several members of the Trump campaign see a few who could pose a threat to his re-election, and are in the early stages of building out their strategy for attack.The bottom line: The three candidates that seem to concern the Trump campaign most are Joe Biden, Kamala Harris and Beto O’Rourke. That's in no particular order, and you’ll get a different answer depending on who you talk to.Why they're worried:Joe Biden: Several Trump advisers think Biden is best positioned to take back the white Rust Belt voters Trump carried in 2016 and make purple states like Michigan and Wisconsin far more dangerous for the campaign.“A guy who loves his guns and God is not voting for Kamala Harris. But he would vote for Joe Biden. He’s a lot harder for them to demonize,” a former Trump campaign staffer told Axios.And despite what he says about Biden in public, Trump respects him the way he respects House Speaker Nancy Pelosi, the former staffer said: “He looks at Biden as that elder statesman, the guy that can connect to the working class voter.”“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,” a Republican operative close to the campaign said. 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 easily cast his attacks as racist and sexist. "With Kamala ... I don't know what she does to the young, but more broadly to the African-American vote. If they come out in huge numbers that's a challenge," a Trump campaign adviser said.The other side: Though several aides admit she’s a winner when it comes to identity politics, they also 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 News last week that Republicans shouldn't underestimate O’Rourke. That is a view shared by a number of people in Trump’s inner circle.He doesn’t have a long resume or a huge amount of experience, one Republican strategist said, but he’s got the X factor that lets you capture the media narrative. He’d also be a generational foil to Trump.“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 was the nominee, he would also force the White House to spend a lot of money in Texas, which they don’t plan to do.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,” the Republican operative said. The runners up: Several aides close to the president said 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. “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," Kayleigh McEnany, press secretary for the Trump campaign, told Axios.