"The charges against President Johnson stemmed from the Tenure of Office Act of 1867. This law, which the Supreme Court declared unconstitutional in 1926, required the president to get the Senate's permission to remove any officeholder whose appointment it had to confirm.

Johnson was so angered by this challenge to the power of the president that he tested the law by suspending Secretary of War Edwin M. Stanton, the only Radical sympathizer in his cabinet, and replacing him with Ulysses S. Grant.

Eleven articles of impeachment were brought, charging Johnson with unlawfully removing Stanton and violating the Reconstruction Acts. The House of Representatives passed a resolution impeaching Johnson by a vote of 126 to 47."

http://www.pbs.org/newshour/impeachment/timeline/johnson.html

"For all practical purposes" Congress treated Johnson like a foreign occupier of the White House, even denying him the powers to appoint his own cabinet members.

As for the suggestion that the Democratic Party vanished in 1861 I would suggest that the organizers of the Jefferson/Jackson dinners would be profoundly surprised at knowing the Party mysteriously vanished.

Johnson was on the ticket as an attempt at ticket balancing, Republicans neither wanted nor expected him to become President. And the whole affair was partisan to the core.