American history is chock full of near-misses, twists of fate, lucky breaks, and obscure politicians who almost became leader of the free world. The vice presidency has been vacant 18 different times, sometimes for years at a time.

Every one of those vacancies represented a constitutional crisis that wasn’t dealt with until the passage of the 25th Amendment in 1967. There was no way to fill a vice presidential vacancy, and it’s not clear whether Congress has the authority to call off-year presidential elections. With no president or vice-president, the very legitimacy of our government could have been put to the test.

Other times, presidential elections came down to razor-thin margins that hinged on just one state or a few thousand people. And it’s not just Al Gore in 2000.

Each represents a fascinating alternate history of America: what would things be like now if any of them had wound up sitting in the big chair? Would the course of America have been utterly changed? Or does it even matter who the president is, given how much our system relies on checks and balances?

Aaron Burr, 1800: The Founders didn’t anticipate the rise of political parties, and wrote a massive flaw into the Constitution: each member of the Electoral College could cast two votes, with the winner becoming president, the runner-up vice president. This system (which would soon be scrapped by the 12^th Amendment) almost immediately fell apart, when Thomas Jefferson and Burr, his running mate, each wound up with 73 electoral votes. With no winner, the election went to the House of Representatives, and after 36 ballots, Jefferson won.

Willie Mangum, 1844: For most of the 19^th century, the next person in line to succeed the president was the president pro tempore of the Senate. They would become acting president until a special election was held the November of the year the two vacancies occurred, except in a regular election year.  In theory, an off-year election could have forever changed the American electoral calendar.

The first brush with an acting presidency was when John Tyler, who took office when William Henry Harrison died after a month, was nearly killed during a ceremonial cruise on the USS Princeton in July 1844. Tyler had lingered below decks, supposedly to hear his son-in-law sing a ditty, while his cabinet went up to see a massive gun fired. The cannon blew up, gruesomely killing several cabinet members. If Tyler had gone up top with them, he likely would have died as well, making president pro tempore Willie Mangum acting president.

Even if Mangum had become acting president for a few months, little would have changed in American history. Tyler had a brief run for re-election in 1844, but dropped out. And Mangum wasn’t popular enough to garner nomination on his own.

David Rice Atchison, 1849: President-Elect Zachary Taylor and Vice President Elect Millard Fillmore refused to be sworn in on a Sunday, meaning that on March 4, 1849, the United States had no sworn-in president. This left President Pro Tempore David Rice Atchison in a kind of acting president role, though there are constitutional arguments that the president becomes president the moment the previous president’s term ends, even if they aren’t sworn in. Regardless, Atchison took no presidential action, and is said to have slept most of the day.

Hannibal Hamlin, 1865: The affable Senator from Maine was Abraham Lincoln’s vice president from 1861 to 1865, when he was removed from the ticket in favor of southerner Andrew Johnson. Just 42 days into Lincoln’s second term, he was assassinated, and Hamlin missed his chance. A Hamlin presidency would have been a substantial change from Johnson, who favored a soft touch with former Confederate states, and was sympathetic toward slaveholders.

Hamlin was a staunch abolitionist, like Lincoln, and it’s likely that his presidency would have been a continuation of Lincoln’s nascent policies toward the defeated South. It could have been a much smoother reconciliation, avoiding the tumult and institutionalized racism of Reconstruction under Johnson.

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Lafayette S. Foster, 1865: The plot to kill Lincoln also involved killing Vice President Johnson, but the man tasked with killing Johnson lost his nerve. He likely would have succeeded had he tried, leaving President Pro Tempore Foster Acting President until a special election in November of that year. With just a few months in office, it’s not likely that Foster would have done much, and an 1865 election would have been an opening for Ulysses S. Grant to take office three years early, and bring a harsh retribution upon returning Southern states.

Benjamin Wade, 1868: Johnson’s term was marred by controversy over post-Civil War Reconstruction. It nearly ended early when Johnson was impeached, and tried in the Senate. He escaped removal by one vote, otherwise, president pro tempore Wade,a moderate Republican who staunchly opposed Lincoln’s conduct of the Civil War, would have succeeded him. It’s speculated that Johnson was acquitted because radical Republicans hated the moderate Wade as much as they hated the Democrat Johnson.

Samuel Tilden, 1876: Democrat Tilden and Republican Rutherford Hayes turned the election of 1876 into the most contentious in American history, one marred by voter intimidation, dirty tricks, and violence. Tilden easily won the popular vote, but was one vote short of an electoral majority, with 20 votes being contested. After months of negotiations and hearings, Hayes was awarded the 20 votes, in exchange for the withdrawal of federal troops from the South, prematurely ending Reconstruction.

What would a Tilden presidency have looked like? It’s hard to say, but Tilden was known for being a corruption fighter and reformer, having taken down the both the Boss Tweed corruption ring and a ring of New York politicians who had been plundering the building of the state’s canal system. But he was also an advocate of peace with the South, and opposed both going to war over slavery and Radical Republican harshness toward the Confederacy. So he might have been more preoccupied with cleaning up the messes left by Grant’s corrupt administration than with putting the country back together.

Charles Evans Hughes, 1916: The election between incumbent Democrat Woodrow Wilson and Republican Hughes came down to a tiny margin in California. Critically, Hughes had visited California, but snubbed the state’s popular governor, Hiram Johnson, when they were staying at the same hotel. A series of misunderstandings kicked off from there, the snub hit the papers, and turned many Republicans off from Hughes. Wilson won California by about 3,000 votes, and with it, a second term.

Hughes wouldn’t have been able to keep the US out of World War I, but was much more progressive in his policies and beliefs than Wilson. And it’s likely that the US would have played a bigger role in postwar international unity under Hughes, since Wilson’s massive stroke ensured the Senate didn’t ratify the League of Nations.

John Nance Garner, 1933: A month before Inauguration Day, 1933, president-elect Franklin D. Roosevelt was riding in a car in Miami with Chicago mayor Anton Cermak. An anarchist shot at the car, killing Cermak, but FDR was unharmed. Under the 20^th Amendment, ratified just weeks earlier, if a president dies before inauguration, the vice president, in this case Garner, would be inaugurated instead.

Garner and Roosevelt differed sharply on economic issues, and it’s likely that a Garner presidency would have seen a massively curtailed New Deal. It also means that the US would likely have had a different president when World War II broke out – sending history spiraling out into an even murkier future.

Henry Wallace, 1945: Wallace succeeded Garner as FDR’s vice president for his third term in 1940, but was removed before the election of 1944. Democratic bosses felt he was sympathetic toward the Soviet Union (which he was), and replaced him with Harry Truman. Roosevelt died 82 days into his fourth term.

If Wallace had been renominated, it would have put an avowed pacifist in charge of wrapping up the Second World War, including the decision to use the atomic bomb. It also might have led the US to make major concessions to the Soviet Union during postwar negotiations.

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Carl Albert, 1973/1974: When Richard Nixon’s vice president, Spiro Agnew, resigned due to a tax evasion scandal, it represented the first test of the 25^th Amendment, recently ratified to allow vice presidential vacancies to be filled. But before Gerald Ford was confirmed to be VP, the office was still vacant for 57 days, and if Nixon had died or resigned during that time, Speaker of the House Albert was next in line, as the line of succession had been reorganized in 1947. Indeed, the impeachment process for Nixon actually began during the time when Albert was Speaker.

Albert filled the same position a year later, this time for over four months, after Nixon resigned and Ford replaced him.