On the American side, a varied cast of characters was fielding overtures and proposals from Russians or pro- Russian Ukrainians during the campaign and transition, including: Mr. Gerson; George Nader, the Lebanese- American with Trump campaign connections; Paul Manafort, Mr. Trump's former campaign chairman; Michael D. Cohen, the president's longtime fixer and lawyer; and Erik Prince, the Blackwater founder and brother of Betsy DeVos, Mr. Trump's pick for education secretary.