Andrew Cohen is the author of Two Days in June: John F. Kennedy and the 48 Hours that Made History.
An adjective often used to describe the rise of Donald Trump is "unconventional." Who could have seriously imagined that a strongman, nativist, egoist, populist and plutocrat would be the nominee of the Republican Party?
After Super Tuesday, that's likely. His success lies in his artless frankness, his indifference to facts, his defiance of orthodoxy. He is the opposite of Hillary Clinton, the consummate insider and tutored policy wonk.
In the general election, though, what made him the standard-bearer of the Republican Party will yield to other factors. These will make Donald Trump the weakest Republican nominee in a half-century. In a year when all three branches of government (the presidency, Congress and the Supreme Court) are in play, there are reasons for Democrats to cheer.
Age: On Jan. 20, 2017, Mr. Trump will be 70. He would be the oldest president to take office in American history; it is now Ronald Reagan, who was 69.
Traditionally, age has not mattered in choosing presidents. But in nominating a septuagenarian, the Republicans lose one of their strongest arguments against Ms. Clinton: generational change.
The other two Republican contenders – Marco Rubio, 44, and Ted Cruz, 45 – represent that. Mr. Trump does not. Ms. Clinton will be a comparatively youthful 69 in January.
Demography: Mr. Trump is a white male in a country in which white males are 31 per cent of the population and falling. Given the changing composition, any Republican would have trouble winning without taking a significant share of black and Hispanic America, which comprise 30 per cent of the population and rising.
Mr. Trump is least able to make that appeal. He is not Hispanic like Mr. Cruz and Mr. Rubio or black like Ben Carson. While he has mobilized the party's base, he has not broadened it. Without currency beyond white men, the party can't win. Ms. Clinton isn't the novelty as a woman she was in 2008 but, against a clownish Mr. Trump, she will draw moderate independents and alienated Republican women.
Experience: Mr. Trump is an entrepreneur, showman and anti-politician promising to blow up Washington. Historically, though, Americans have rejected outsiders on horseback. While political experience is less important than it was – Barack Obama came to the White House with only four years in national office – it remains a measure of credibility.
Since 1900, only Herbert Hoover (1928) and Dwight Eisenhower (1952) became president without holding elective office. Hoover was a wealthy engineer who was Secretary of Commerce; Eisenhower was a five-star general who commanded Allied Forces in the Second World War.
Mr. Trump has not served in cabinet or the military (though he attended New York Military Academy). His career in real estate and reality television included no public service – in contrast to Ms. Clinton, who was first lady, senator and cabinet secretary.
Geography: Presidents always carry the state where they were born or live. Since 1900, every president has won one or the other.
Mr. Trump lives in New York, as does Ms. Clinton (born in Illinois). New York has not voted Republican since 1984. Mr. Trump will bring no personal geographical electoral advantage that Marco Rubio (Florida) and Ted Cruz (Texas) would.
Party: Unity matters. The Democrats lost office in 1968 and 1980 because of a wasting nomination fight. The Republicans lost in 1964 because of an ideological schism.
Ms. Clinton will lead a united party with the blessing of an incumbent Democrat who enjoys high public approval. Mr. Trump will lead a party threatening to come apart. Angry Republicans could walk out of the convention in Cleveland – as Dixiecrats left the Democratic Party in 1948 – and leave the party entirely. Or, the party could split, as the Democrats did in 1860, electing Abraham Lincoln.
Continuity: While it seems unusual for the party that has been in the White House for eight years to win a third term under new leadership, it has happened in the modern presidency – in 1908, 1928, 1948 and 1988. With a strong economy and a popular outgoing Democrat, timing may favour Ms. Clinton.
Of course, accidents happen in a campaign. But Mr. Trump, the last gasp of a fading United States, is not only running against Ms. Clinton. He is running against history.