What constitutional action starts the process by which a president is removed from office?

In order for a President to be removed from office he/she must first go through the process of Impeachment. In the United States entire history only two Presidents have been impeached and they are Bill Clinton and Andrew Johnson; this goes to show it is pretty rare to get impeached out of office. In order for impeachment two thirds of both the House and Senate have to vote on whether or not the President can continue to assume his duties, if not the Vice President would take over until the next Presidential election.

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