'Whistleblower' has significant ties to a Democrat running for President
...I suspect it may have been Biden. If it was the report has certainly harmed Biden by highlighting his alleged corruption. If it was someone competing with Biden they may have wanted to use this to pull him down.
Now, however, there is word of more evidence of possible bias on the whistleblower's part. Under questioning from Republicans during last Friday's impeachment inquiry interview with Atkinson, the inspector general revealed that the whistleblower's possible bias was not that he was simply a registered Democrat. It was that he had a significant tie to one of the Democratic presidential candidates currently vying to challenge President Trump in next year's election.
"The IG said [the whistleblower] worked or had some type of professional relationship with one of the Democratic candidates," said one person with knowledge of what was said.