If Vice-President Andrew Johnson had been shot and killed walking up Tenth Street to visit President Lincoln on assassination night, the law of succession in 1865 would have handed the acting presidency to Senator Lafayette S. Foster of Connecticut until the secretary of state could arrange a new election.
Sen. Foster was president of the Senate pro tempore.
If John Wilkes Booth’s plan he worked, it would have thrown the government into chaos because the president, vice-president and secretary of state would have been dead.
Source: John Wilkes Booth’s Escape Route, Notes by James O. Hall, published by The Surratt Society