WASHINGTON — Speaker Nancy Pelosi has been quietly sounding out top allies and lawmakers about whether the time has come to impeach President Donald Trump, a major development as several moderate House Democrats resistant to impeachment suddenly endorsed the extraordinary step of trying to oust the president. Pelosi, according to senior House Democrats and congressional aides, has been gauging the mood of her caucus about whether they believe allegations that Trump pressured a Ukrainian leader to investigate a political foe is a tipping point.