Congress can not indict or convict a someone with of a crime. Just because they are investigating a crime doesn't make it a criminal proceeding.
If Nixon was actually impeached and convicted, he still wouldn't have a criminal record until the DOJ indicted him and he either plead guilty or was found guilty by a trial.
Like the FBI, they can recommend someone get indicted. But they can't actually charge anyone with a crime.
Thanks Captain Obvious. The point is it was based on criminal activity of which Nixon was involved in, which was of primary relevance to their oversight ability. What criminal act is this current impeachment based on? Oh right, 3 years and not a shred of evidence of criminal activity on Trump's part has been presented. Even Nixon had a vote for his subpoenas to be issued. This investigation and resulting subpoenas are a farce.