Fact-check.org is run and funded by the Annenberg foundation.

Annenberg bankrolled Bill Ayers and Barack Obama in Chicago. The company was started by a conservative but his heirs have made it into a highly liberal foundation.

One of the other big founders of fact-check.org is Facebook. You can speculate as to which side big tech roots for.

While factcheck.org is not as nakedly partisan as politifact or other similar slimeball services, and tries to deflect criticism by admitting dem whoppers, it leans left.