What is opposition research?

Donald Trump Jr.'s admission that he met with a Russian attorney in hopes of getting dirt on Hillary Clinton is putting the once-shadowy field of opposition research into the spotlight. Details mined from opposition research have played prominently in past campaigns. Think John Edwards' $400 haircut. A researcher for then-Sen. Barack Obama's campaign stumbled on the expenditure in a campaign finance report and passed it along to Politico.

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