In 1980, Republican presidential hopeful Ronald Reagan planned to make an unprecedented move in American politics. Shortly after announcing his candidacy, he sat down with advisers and hashed out a plan for a running mate: Gerald Ford, the President who held office from 1974 to 1977.

When broadcast journalist Walter Cronkite asked Ford point-blank if he and Reagan were considering what Cronkite termed a “co-presidency,” Ford dodged the question with a non-denial; Reagan was taken aback that Ford would consider the arrangement to be one of equal influence. The idea stalled out, and George H.W. Bush became Reagan’s vice president.

If the issue had progressed, Reagan would have skirted close to a Constitutional impasse. What happens if your vice president, who is a former president, re-enters the office of the presidency if the commander-in-chief is indisposed? And is such a move even constitutionally possible?

The 12th Amendment, which was ratified in 1804, directs that no one “ineligible to the office of President shall be eligible to that of Vice President of the United States.” The 22nd Amendment states that “[n]o person shall be elected to the office of the President more than twice." (Thank you, Franklin Roosevelt.) While this means the not-quite-one-term Ford would have been fine, it does mean a president who has served two terms couldn’t possibly be selected as a running mate.

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Or could they? Some Constitutional scholars have argued it's possible. Columbia University law professor Michael Dorf explored the topic in 2000, when the exiting Bill Clinton was being earmarked as a running mate for Al Gore. Clinton, Dorf argued, wasn’t “ineligible,” just not electable—a big difference. If a former-president-turned-veep were to re-take office, it wouldn’t be due to an election, it would be due to the departing president’s death, removal, or resignation. The same would hold true for serving more than two terms—the third would be a succession, not an election.

A 2006 Washington Post article delved into the question further, finding three lawyers and a federal judge who agreed—with Hillary Clinton eyeing the Oval Office, they asserted that nothing in the Constitution would prohibit a two-Clinton ticket (the issue of them being from the same state is a discussion for another time). Others, however, stated taking “elected” at face value is being too literal, and that the spirit of the amendment was to prevent anyone from holding office for more than two terms regardless of how they arrived there.

The latter argument was supported by Bill Clinton, who has been fielding questions of his possible vice presidency alongside Hillary. Talking to David Letterman in 2007, Clinton said that “I just don’t believe it’s consistent with the spirit of the Constitution for someone who’s been president twice to be elected vice president … I don’t think it’s right and I wouldn’t want to do that.”

Grilled by Mario Lopez in 2015, Hillary Clinton echoed the sentiment, saying that her husband is “not eligible … it would not be possible for him to ever succeed to the position.”

An equal amount of confusion existed back in 1960, when Dwight Eisenhower jokingly floated the idea of running for vice president. He didn't, of course, for the same reason few presidents have ever sought office after stepping down: after ruling the free world, no one wants to accept a demotion.