Sarah Palin is the product of a design flaw -- the unintended consequence of the founders' decision to create the vice presidency.

For two centuries, presidential nominees have used the office to balance the ticket by naming a running mate from a different region, or one who speaks with a different ideological accent to a specific constituency. This means that a president's death generates a double shock: The nation not only mourns a fallen leader, it must deal with a replacement who might push politics in a new direction.

Teddy Roosevelt -- who replaced William McKinley when he was assassinated in 1901 -- may have been a great progressive president, but he had been named as vice president by the arch-conservative McKinley simply to carry New York. The country elected a right-winger but ended up with something else entirely.

Similar perverse logic led Abraham Lincoln to choose Andrew Johnson as a running mate. Lincoln knew that Johnson was a racial conservative, but he was more interested in carrying Tennessee. This tragic blunder clouds Lincoln's claim to greatness. When Lincoln was killed, Johnson's bitter opposition to Reconstruction helped poison race relations for generations.

Recent elections have lulled us into a false sense of security. Both Bill Clinton and George W. Bush nominated like-minded, known-quantity running mates, as has Barack Obama this time around. But John McCain's surprising choice should lead us to think again. Mexico and France see no need for a vice president. We should designate the secretary of State to be in charge until a special election can be held to replace a president.

This isn't a question on which the founders deserve any deference. They designed their system for a very different political world.

Their electoral college aimed to give the power to choose the president to wealthy, important men in each state. But politics was emphatically local in the 18th century, and the founders feared that each state's electors would cast their ballots for a favorite son -- depriving the leading candidate of a majority.

To solve this problem, they hit upon an ingenious scheme. The original Constitution gave presidential electors two votes, not one, and provided that they could only vote for one nominee from their own state. The idea was that electors would use one vote to flatter a local favorite and the other to select a national leader like, say, George Washington, giving him a strong majority.

But alas, the two-vote system could be sabotaged. Electors could simply vote for their favorite Joe Schmoe and cast a blank second ballot, thereby maximizing Schmoe's chance for success. Enter the vice presidency, a consolation prize for favorite sons (or whoever polled second in the electoral college). It was meant to assure the election of a proper president; providing a replacement executive was a distinctly secondary objective.

This clever scheme did not survive the rise of political parties.

By 1800, electors followed the instructions of the national Democratic-Republican and Federalist parties, generating a crisis. The Democratic-Republicans won the election, and their 73 electors all wrote the names of Thomas Jefferson and his running mate, Aaron Burr, on their ballots, without having the power to say that they wanted Jefferson to be president. This created a tie for the top spot, and Jefferson and Burr were obliged to compete for the presidency in what turned out to be a bitterly contested runoff in the House of Representatives.

After the smoke cleared, Congress and the states merely tinkered with the election system. Their 12th Amendment simply told electors to cast one ballot for president and one for vice president. This made a repeat of the Jefferson-Burr crisis impossible, but it paved the way for the vice presidential shocks that have redirected our national history.

If McCain wins the presidency, we can only wish him a long life. But however the race turns out, we should recognize that the founders didn't have the slightest idea that the vice presidency would episodically explode in our face, and it's about time we fixed it.

Ackerman is a professor of law and political science at Yale and the author of "The Failure of the Founding Fathers."