Biden seems to get under Trump's skin more then the other announced candidates. When Biden started out by criticizing Trump's Charlottesville "good people on both sides" remarks, Trump took the bait and claimed his remarks were "perfect" and that he was referring to people defending the Robert Lee statue, and not the neo-Nazis chanting "Jews will not replace us". And he said everybody knew that. Except they didn't know that at all, and he was referring to the neo-Nazis.
It's been clear for awhile now, IMO, that Biden worries Trump more then any of the other candidates. I think Trump understands that Biden can cut into Trump's working class support in the states Trump turned red in 2016.
When the International Association of Firefighters gave a full throated endorsement of Biden, Trump went into tweetstorm mode again. Justified or not, Biden worries Trump. I think Trump would prefer a candidate he can use socialism as a sledgehammer against in the general election.
Also telling is that reports indicate the DHS, when communicating with Trump himself, stays away from the topic of Russian interference in the 2020 election. That one cannot raise the issue with Trump because it affects his ego. So, he is not exactly into cybersecurity and the 2020 election, despite taking an oath to protect and defend.
https://www.sltrib.com/opinion/commentary/2019/04/30/jennifer-rubin-how-biden/
"I'll never get the support of Dues Crazy union leadership, those people who rip-off their membership with ridiculously high dues, medical and other expenses while being paid a fortune. But the members love Trump," one tweet said.
"The Dues Sucking firefighters leadership will always support Democrats, even though the membership wants me," read another a few minutes later. "Some things never change!"