I understand what the methods were in obtaining the data and phone conversations. The problem is when federal agents were tasked to focus on Trump and his team, create reports and then distribute these far and wide with all parties and information unmasked..
Under federal law, there is wide lattitude allowed in impeachable evidence. Even evidence that would be illegal to present as direct, is allowed to impeach claims made. Ex: use of illegal wiretaps, illegal eavesdropping or stolen documents So when Trump and associates said they had no conversations with the Russians, they were merely gathering evidence with which to impeach that claim. And would probably be presentable if the case went to federal court.