When asked about the impact of a president having a physical ailment, White explained that the 25^th Amendment created guidelines for a president to temporarily step away from his position, including in the case of a medical emergency.  This has come into play during 1985 colon cancer surgery for Reagan and when George W. Bush underwent colonoscopies in both 2002 and 2007.  Each time, the president wrote a letter to both the vice president and Speaker of the House, relieving himself from duty during surgery. He then submitted another letter to reclaim authority after the procedures were finished. 

However, according to White, the amendment doesn’t provide clear guidance on what would need to be done in the event of an unplanned medical emergency, such as a stroke or heart attack.  "The 25^th Amendment is also not very thorough on a declaration of 'disability,'” White noted. “This creates the potential for political intrigue, as the vice president and Speaker of the House are responsible for declaring a president to be out of commission. The intent was to have continuity in governance in the nuclear age, but it isn't perfect." 

What’s not clear is what level of disability would become an issue today.  Sen. Joe Biden's health was extensively researched during the 2008 campaign before he was selected as Barack Obama’s running mate, and it was decided that a brain aneurism he had suffered two decades earlier should not prevent him from running for vice president.

"The only time that the process didn't work, it seems, was with Dick Cheney," White noted. Cheney originally headed the selection process to pick a running mate for George W. Bush's, but ultimately threw his own hat in the ring, despite having a history of heart attacks.

White did present an interesting question regarding the definition of disability under the 25^th Amendment: What if President Kennedy had lived after being shot?  If he had survived, he would have been severely incapacitated, and even if he had suffered only the initial shot through his throat, he might have been unable to speak.  In 1957, President Dwight Eisenhower suffered a stroke and temporarily lost his ability to speak; Dr. Jerrold Post, director of the political psychology program at George Washington University, was quoted in a 2008 piece on The Daily Beast suggesting that this "raised the specter of a president who was unable to communicate." 

This question remains, as no president has been in such a situation in many decades.  That said, the health of the president and disability could become a touchy issue, although, as White pointed out, "FDR was proof that being bound to a wheelchair did not impede his ability to govern." More likely, some form of cognitive decline would qualify as a reason to question whether a person could continue to serve as president.

This issue of cognitive decline came to prominence in 1984, while President Ronald Reagan was campaigning for his second term.  During the first debate with his opponent, Walter Mondale, Reagan was reported to look "completely out of it," according to White.  Reagan's advisors feared that age might become more of an issue, but in the second debate, Reagan tackled the issue head-on, joking about Mondale's "youth and inexperience" while deflecting concerns about his rising age.