President Johnson constantly clashed with Congress, which wished to impose a significantly more radical version of reconstruction on the South. Finally, the radicals in Congress decided to try to impeach and indict Andrew Johnson.
The impeachment vote passed in the House. The Senate voted 35 for and 19 against conviction; one vote short of the needed 2/3rds majority.
From the moment Lincoln was assassinated the stage had been set for clashed between Congress and the new President, a Southerner from Tennessee. Johnson had been the only southern senator to remain loyal to the Union. Lincoln had picked him to be his Vice Presidential candidate in act of reconciliation, with Union victory all but certain in 1864 he wanted to show that he intended to rapidly reintegrate the South into American life. His policy was that Southern States only had to renounce slavery in order to reenter the Union. Johnson continued Lincoln's policies but without the authority that Lincoln had. The Republicans in Congress believed that a harder line should be taken with the southern states and that Congress and not the President should be responsible for the policy relating to the Southern States. The clash was ongoing.
In 1867, Congress passed the "Tenure Act." Under this act the President was forbidden to remove certain public officials without the consent of the Congress. In the beginning of February, President Johnson removed Secretary of War Stanton from office. Johnson felt that Stanton, who was a radical Republican, was undermining the policies of the president.
In response to Johnson's actions, on February 24th, the House voted 126-47 to impeach President Johnson, for "high crimes and misdemeanors." The senate allowed Johnson only 10 days to prepare his defense.
On May 16th, the first of the articles of impeachment came to a vote. The Senate vote was 35-19; one vote short of the two thirds needed for a conviction.
Seven Republican senators joined the Democrats in voting for Johnson's acquittal.