I’m still trying to get my arms around why it should be a problem to ask ANY country to help us investigate corruption — regardless if the person is running for office or not. This is a far distance from asking a country to invent intelligence to overturn an election.
...which, by the way, is more corruption.
It's called the "Treaty with Ukraine on Mutual Legal Assistance in Criminal Matters", Treaty Document 106-16. Approved in 1999 under President Clinton. In Senate records and downloadable as a PDF - 31 pages.
It authorizes mutual assistance in criminal investigations.
Trump is following the law, not violating it.