I don't think anyone is "underestimating" Trump really. Voters have spoken in 3 different cycles now. And have largely rejected both Trump and his band of crazies. Some are close races yes, a few made it through in deep red states and counties. But, by large it isn't just a "feeling" that we are sick of Trump. It has been shown at the ballot box.
Even with Biden's low approval rating he still had one of the better midterms in decades for any incumbent. And if not for New York it looks like Dems could have kept the House too. Even the GOP are sick of Trump and its obvious. The difference is they created a monster gave him the keys to their fortunes and can't control him. So yes he can waltz right into this race like the Impeachment committee told them would happen.
What is his message going to be that will resonate now that didn't before? Ivanka and Jared are even out this time and want nothing to do with politics. Has he changed? Is he going to be less racist? Just weeks ago he was lamenting the harsh treatment of his Jan 6 mob in jail and courtrooms. So he certainly isn't sorry he sent them to attack the capital. He isn't sorry he took Confidential documents to Mara lago and had to get raided to get them back. He learned and reflected on his years of election denial? NOPE. Hasn't changed still lying spreading conspiracies.
There is visceral HATRED for Trump that motivates voters against him. Meanwhile his supporters he let die from COVID and tainted them thinking elections are rigged anyway.