alexander vindman raised concerns after listening to the call. he, too, is slated to appear on tuesday. >> there is no one star witness that this whole case will rise or fall on that we know of right now but i think vindman is another brick in the wall that house democrats are trying to build. >> reporter: former representative to ukraine kurt volker defended the president's actions in closed-door testimony saying there was no leverage implied. >> there was no linkage and further testimony will confirm that multiple times. >> we have text messages from volker and sondland. >> reporter: volker's private text message the morning of the july 25th call revealed ukrainian officials pushing for a white house meeting. volker writing to an aide for zelensky, assuming president z convinces trump he will investigate, get to the bottom of what happened in 2016, we'll nail down a date for the visit