"Collins is a dimwit."  I see.  Well, despite your ridiculous assertion about Senator Collins, she is a sitting United States Senator, with all of the power that comes with that elected office.  That includes the power to request documents and to receive them, in the course of an investigation after following due process.

The Fourth Amendment states:  "The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized."

Tax returns can be discovered during an official investigative action by following due process.  You should probably consult your attorney to learn more about that.  It's settled law.