With Nixon, the House specified a crime, then proved that he committed it.
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With Trump, the House made broad accusations, then found a bunch of people who disagreed with how Trump has executed foreign policy and called them witnesses.
It's not broad at all. The charges against him are that he made foreign policy decisions for his own personal enrichment using non-government personnel and back channel deals. And the Republicans defense isn't that Trump didn't do it's "yeah he did it, but so fucking what?" It's well against the law to use the office of the president and the federal government as your own personal business, but we are so used to it by now that we've become numb to it.
Personal enrichment? His private assets have been hemorrhaging capital since he announced his candidacy. The charges are Obstruction of Congress and Abuse of Power. Neither of them hold water. Congress can subpoena whoever they damn well please, and POTUS can say no for just as long. Think Executive Privilege doesn't apply? Get SCOTUS to break the stalemate. Abuse of power in Ukraine? Nope, it's the Executive branch's prerogative to investigate cases of corruption involving Americans, especially when the corruption includes a high office, blatant withholding of aid conditional on an anti-corruption statesman being fired, and when the current administration offers no threats or rewards conditional on investigating how the hell that was allowed to happen.
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With Trump, the House made broad accusations, then found a bunch of people who disagreed with how Trump has executed foreign policy and called them witnesses.