there is a famous scene from witness for the prosecution (1957) which you likely never has seen but know even so 'cause has been done in variations a thousand times.
"the question is, frau helm, were you lying then, are you lying now, or are you not in fact a chronic and habitual liar!"
(will check internet)
is verboten to attack the character o' a witness not 'cause we want politeness in court but 'cause to impugn character o' witness is deemed irrelevant and has a tendency to prejudice that witness in the eyes o' a jury. exception: propensity for falsehood is always relevant, which is one reason no lawyer would ever put trump on the stand to act as a witness in his own defense. regardless, the aforementioned scene, plays out everyday in courts 'cross the land, albeit with far less drama.
impeach a witness as to their tendency for mendacity is relevant and allowable under the rules o' evidence in every US state as well as fed courts. however, is not some kinda objective standard. judge doesn't instruct jury to dismiss witness testimony if the witness lied 'bout their age on a facebook profile. assuming the witness offers a decent explanation which convinces the jury/judge they is telling the truth, such as speaking against their own interests, then a finder of fact may decide the testimony offered is reliable enuff.
keep in mind that it is illegal to lie to the feds in most instances. by admitting he/she lied to feds, the witness exposed self to legal jeopardy. have witness explain why he/she were afraid o' speaking truthful to the feds may not be in trump's best interest, so the degree to which prospective defense attorneys seek to impeach on truthfulness is a tactical consideration which is all kinda schrödinger's cat at the moment.
HA! Good Fun!