Joe Ravi [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
The process of Presidential Succession has always been problematic and heavily debated but the constitution has the answer. James Madison, the father of our founding documents, heavily criticized the original act in 1792. Madison criticized it for not being constitutional and being a move drowned in politics. At the time no one in Congress wanted to see Thomas Jefferson or Alexander Hamilton become the President due to an unexpected passing of the President.
With all this in mind, it is not unreasonable to think that if President Trump and Mike Pence were removed from office the Republican Party would take the fight to the Supreme court. Unfortunately for Pelosi, the evidence is on the side of the RNC and a Trump Appointee would likely be put in the position.
With all this out of the way, here is the official order of Presidential Succession:
1.) Vice President
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Source: TTN staff