NANCY PELOSI HAS NOT had a simple job since the Democrats won back the House last year, having to, most prominently, swat away an ill-fated attempt to replace her as Speaker, navigate uncharted political waters with the emergence of the progressive “Squad,” and try to negotiate with Donald Trump. But on no single issue has she faced a more persistent drumbeat from assorted members of her caucus than the impeachment of the president, whom her entire party regards as a criminal but whom far fewer have believed it wise, politically, to prosecute. Suddenly, though, almost her entire caucus is behind her, and Pelosi’s months of caution (years, really, since Maxine Waters and Al Green first started calling for impeachment) look something like strategic prescience. It’s true that the Ukraine story…