The Crisis of Sucession: 1865

[MASHED POTATOES. VOTE!]

It's very interesting, but completely incorrect.

According to the Presidential Succession Act, the President pro tempore of the Senate would serve as Acting President until special election.

From 1792 to 1886 the line of succession was as follows:

1. Vice President
2. President pro tempore
3. Speaker of the House

Cabinet officers were not included to the line of succession until 1886.

Of course, due to events in your TL, Congress could amend this in order to allow Stanton to serve as Acting President, but the law is not retrospective.

Therefore, Senate PPT Lafayette S. Foster would succeed Lincoln.

[MASHED POTATOES. VOTE!]

If I may suggest an easy way to resolve this problem:

Foster is succeeding Lincoln as per law, but since political establishment want Stanton to become President, Congress may quickly amend the act and Foster would simply resign in Stanton's favor. Legitimacy is saved.