You opened the door.
Trial lawyers live in fear of that phrase.
When a trial starts, both sides know what the allegations are. Both have had enough discovery to know what the adversary will try to prove. Just as significantly, both know what their own vulnerabilities are. A litigator spends his pretrial time not just laying the groundwork for getting his own evidence admitted by the court; each side works just as hard on motions to exclude embarrassing or incriminating testimony — evidence that would be damaging to that side’s position but that a court may be persuaded to exclude because it is not clearly relevant.
For an advocate, it is a coup when the judge rules that harmful testimony is excluded. But such rulings always come with a warning label: Don’t open the door. That is, don’t do anything that makes the otherwise irrelevant evidence relevant.
President Trump’s impeachment trial has a Biden door. Adam Schiff has thrown it wide open.