What if Grant didn't turn down Lincoln's invitation to Ford's theater that fateful night?
Suppose the assassination still happens, but Grant is able to injure and capture or kill Booth while sustaining non-fatal injuries.
Fast forward to 1868 with Grant replacing Edwin Stanton as Secretary of War. With all the stress and burden of the office and Johnson's fight with congress, Grant's health takes a downturn due to his injuries. He doesn't die, but swears off any further political endeavors, at least for the next few years while his health recovers.
Who would be the most likely Republican, and perhaps, National Union (if applicable) presidential candidate?