Andrew Johnson. Not mocking you, just saying. Different guy. Senator from Tennessee, the only senator from a rebel state to remain in Washington, which is why Lincoln picked him as VP. Got impeached for resisting the attempts of the Radical Republicans in Congress to punish the rebel states, though the formal reason was alleged violation of the Tenure in Offices Act.Convince 1/2+1 of the republican controlled house to impeach him and 2/3 of the republican controlled senate to actually agree.
The first part has only been done to a president twice. Andrew Jackson and Bill Clinton. Neither were convicted in the senate.