A simple majority of the U.S. House of Representatives (at least 218 votes) is required to impeach a U.S. President, followed by a two-thirds majority vote in the Senate (at least 67 votes). The number of votes required make impeachment difficult. No American President has been removed from office by impeachment.
As of 2014, there have been four cases when impeachment was seriously considered by Congress. The Presidents involved were Andrew Johnson, John Tyler, Richard Nixon, and William J. Clinton. Both Johnson and Clinton were impeached by the House but were acquitted by the Senate. There were not enough votes in the House to impeach Tyler. Nixon resigned before impeachment proceedings moved to a vote in the House.