This is, by itself a impeachable offense.
I disagree with the reply by paul_lukasiak that the Congress needs to go to court to pursue evidence that the administration is obligated to provide. The administration works for the people, not their own benefit or political expediency, and they have no excuse whatsoever not to come forward with documents and 100% truthful testimony, when subpoenaed by Congress or not, and whether compelled by a court order or not.
But beyond failure to allow testimony or produce documents is itself an impeachable offense, it also allows – in fact requires – a presumption that such testimony and documents would prove malfeasance as any alternative would be in the party’s interest to comply. There’s no right against self-incrimination for impeachment. If one takes the 5th in Congressional testimony, then one has no claim to the office of public trust they hold.
Trump and the others are attempting to avoid that with specious claims of a “hoax” or “witch hunt” or self-proclaimed immunity or privilege, but the onus is on them to prove that claim as legal, not for Congress to go through a lengthy, multi-stage legal process, just to have Trump and his cronies launch some other theory on why they don’t have to comply.
They are guilty because a) the evidence is that they are guilty and b) they are acting guilty. And that’s guilty of violation of the public trust.