The Democrats are coming out of their Chicago convention brimming with confidence. But to win in November, Kamala Harris will have to defy history — because only once in the last 188 years has a sitting vice president been elected president of the United States.

Many vice presidents have gone on to become president. Some, like Joe Biden, ran after leaving office. Others, like Theodore Roosevelt, Calvin Coolidge, Harry S. Truman, Lyndon B. Johnson and Gerald Ford, assumed the presidency because of the death or resignation of the incumbent, and then ran for a full term on their own. Still others, including Al Gore and Richard M. Nixon, tried and failed to succeed popular two-term presidents. (Nixon ran again eight years later and won.)

The only sitting vice president to win the nation’s highest office in the modern era was George H.W. Bush in 1988 — and he was the first to do it since Vice President Martin Van Buren, the “Little Magician,” was elected president in 1836.

Harris will need some magic of her own to pull off the same feat. Bush succeeded where other modern vice presidents failed for one simple reason: Americans wanted a third Reagan term. Today, no one wants another Biden term. (If voters did, then Biden, and not Harris, would still be the Democratic nominee.) Ronald Reagan left office with a 63 percent approval rating. Today, Biden has 57 percent........

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