___ research, source of “dirt” on a political rival Crossword Clue

 The answer to the crossword clue "____ research, source of “dirt” on a political rival" is OPPO. This is a 4-letter word that is a synonym for "opposition". In the context of politics, opposition research is the process of gathering information about a political opponent in order to discredit them or damage their reputation. This information can include anything from their personal life to their political views.

Here are some other words that could be used to describe opposition research:

Smear campaign

Negative campaigning

Character assassination

Dirt digging

Background check

I hope this helps!