The crime that brought Nixon down was the cover up. It was hard to prove he organized or knew about the Watergate break-in but the White House tapes proved he tried to bury the evidence and protect the criminals on his team. Similarly Clinton was impeached for lying about what he did, not the act itself.
From a legal viewpoint the issue is not what Trump said to Zelenskiy but that he and his team tried to hide the evidence.
The crime that brought Nixon down was the cover up. It was hard to prove he organized or knew about the Watergate break-in but the White House tapes proved he tried to bury the evidence and protect the criminals on his team. Similarly Clinton was impeached for lying about what he did, not the act itself.
From a legal viewpoint the issue is not what Trump said to Zelenskiy but that he and his team tried to hide the evidence.