Since 1968, political parties that changed horses in midstream have ended up drowning.

That year, antiwar Democrats forced President Lyndon B. Johnson out of the race. Robert Kennedy ran and was killed. Vice President Hubert Humphrey and Gene McCarthy fought all the way to the convention in Chicago. There, police battered and bloodied protesters in the streets.

Richard Nixon was elected President.

In 1972, George McGovern dropped running mate Tom Eagleton after the media learned Eagleton had undergone electroshock therapy for depression. The already-weakened McGovern never recovered.

Nixon was reelected, and North Carolina elected Jesse Helms to the Senate.

In 1974, Republican Senators persuaded Nixon to resign. That year, Republicans suffered historic midterm losses. Nixon’s successor, Vice President Gerald Ford, lost the White House to Jimmy Carter in 1976.

In 1980, Ted Kennedy fought Carter for the nomination all the way to the convention in New York City. Carter hung on, but the Democratic horse drowned.

Ronald Reagan was elected President.

In 1988, Gary Hart was the Democratic front-runner and looked like a strong challenger to Vice President George H.W. Bush, whom Newsweek called a “wimp.”

Then the press uncovered and obsessed for weeks over Hart’s alleged affair with a model – tame stuff compared to Trump’s tryst with Stormy Daniels, sexual assault and defamation of E. Jean Carroll, and romps with Jeffery Epstein’s 12- and 13-year-old girls.

Hart reluctantly dropped out, and hapless Michael Dukakis lost a winnable race to Bush.

Democrats clamoring for President Biden’s head should ponder that history.