In 2024 a sweeping DOJ investigation had caught multiple members of Congress, including Susan Collins, in a series of corruption cases. Because of the volume of people the case was still pending when Trump took office. He came in and almost immediately the FBI and DOJ dropped all the cases against Republicans including Susan Collins.
Pro Publica has a big firewalled piece on it the Ai summary is that Collins had illegally taken big donations from defense contractors and they had her dead to rights, until Trump just had teh entire thing squashed. This is of course why Collins has been until recently concerned but signing off on things Trump wanted that she clearly didn't like, if she said no, he could just have DOJ reopen the case on her.
Pro Publica has the story