Mitt Romney was heavily involved in the extremely messy divorce of one of his key supporters ... and the Boston Globe is going to court first thing Wednesday morning in an attempt to unseal the court file as well as lift a gag order ... TMZ has learned.

The divorce was between Staples co-founder Tom Stemberg and his first wife Maureen. We're told the divorce battle lasted for years and was extremely ugly.

Sources tell us Romney gave both a deposition in the divorce and testified in the trial. According to our sources, the Boston Globe got a tip that there was "juicy information about Romney" in the sealed documents.