On Sunday evening, Senator Rand Paul (R-KY) told MSNBC's Kasie Hunt that Donald Trump may not run for reelection in 2020, and that if he does, he may face a 2020 GOP primary challenge.

During the interview, Paul said it may be a year or two before Americans will know for sure whether Donald Trump will even be seeking reelection to the presidency:

"Is President Trump running for reelection? I think you won’t know that until you get into sort of – second, third year of his presidency."

Should Trump step down after one term as president, it would be a very unusual move in U.S. presidential politics, but then again Donald Trump has been an unusual presidential candidate and president. Only three presidents so far in U.S. history have stepped down after serving only one term.

James K. Polk campaigned on being a one-term president who would get things done instead of focusing on getting reelected, and he stepped down after serving his term. Calvin Coolidge announced to reporters out of the blue that he would not seek reelection in 1928. And in 1968, Lyndon B. Johnson announced he would not be running for reelection, as he faced growing dissent over the Vietnam War.

Donald Trump, who is 71 years old, not a career politician, and who hates losing, may find a face-saving exit from the political stage preferable to taking the risk of an electoral defeat, or of an electoral victory that would put him back under the demanding pressures and intense public scrutiny of presiding over the U.S. federal government. For someone like Trump it's easy to see how running for reelection may be a lose-lose proposition.

Should he weigh his options and decide to run for reelection, Rand Paul said Sunday that Donald Trump may face a 2020 primary challenge in his own party, which should raise questions as to what Paul's aspirations may be in the coming years. When asked if a primary would be a good thing for the GOP, Paul said:

"I think no one can stop primaries from happening and there could well be a primary that happens."

Paul says he would support Trump for reelection, saying he sees "the glass as half full" when it comes to the Trump Administration. He did however, raise the possibility of a viable challenger who would campaign on ending the war in Afghanistan, saying he would support such a candidate over Trump:

"You know, there would be people that – if we could end the Afghan war, that’s who I would support. But I don’t think that’s going to be an alternative to President Trump."

Could Rand Paul be referring to himself?

Paul is the second major figure within the GOP to question whether Donald Trump would run for reelection in less than a week. On Friday New Jersey Governor Chris Christie told NBC News' Matt Lauer that he is not so sure Donald Trump will run again:

"If he runs again I would support him, yes, but I'm not so sure what will happen. Four years is a long time, and especially for someone who has not spent a lifetime in politics, so I think those years affect him differently So I'm sure the president will make whatever decision is best for him and his family and the country."