Why was watergate a constitutional crisis?

President Nixon lied when he stated quite categorically that he had no involvement in the break-in at the Watergate complex. Ironically, Nixon used to tape telephone conversations, so that is what did for him. 
While the allegations were being investigated, Nixon became like a lame duck, which meant that he lost all credibility. As legal matters take their time, the "Commander-in-chief" was under the cloud of suspicion, meaning that he could not function as President.

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