James David Vance, who grew up in Appalachia and Middle America, is used to being underestimated by the elite. Democrats and the media attack him as “weird,” and the far-right hurls racist remarks at his wife. His new boss, Donald Trump, said, “The choice of vice president makes no difference. ... You’re voting for the president. You’re voting for me.” That isn’t entirely true. Here is why JD Vance matters.

The Constitution doesn’t give the vice president much to do other than preside over the Senate, and should the vote be equally divided, the vice president casts the tie-breaking vote. Lyndon Johnson was the “Master of the Senate” in 1960 and a man who coveted power, so it baffled many when he accepted the nomination for vice president, the “weakest” seat of power in government. When Clare Boothe Luce asked why he took the job, Johnson famously replied, “Clare, I looked it up: One out of every four presidents has died in office. I’m a gamblin’ man, darlin’, and this is the only chance I got.”

Trump has now surpassed Joe Biden as the oldest person ever nominated for president. The odds that he'll finish his term are stacked against him.

And for that reason, the vice president is very important. When you vote for Trump, the oldest presidential candidate nominated by a major party in history, you’re also voting for Vance.

History changes fast when the vice president becomes president. When William Henry Harrison died, Vice President John Tyler seized the opportunity to cement his name in history. At that time, some believed Tyler should only be president until a new election was held or Congress decided otherwise. Tyler’s ambitious assumption of the role set what became known as the “Tyler precedent.”

Tyler’s political views differed significantly from his predecessor and his party. Tyler advocated states’ rights while the Whig Party supported a larger federal system, which included infrastructure projects and a national bank. He vetoed several bills passed by the Whig-dominated Congress, causing his Cabinet to resign in protest and leading to his expulsion from the Whig Party. But Tyler didn’t let that deter him. His ambition for extending America’s territory and securing states’ rights led to the annexation of Texas through a joint resolution, which kicked off a chain of events that culminated with the Civil War.

Tyler isn’t the only vice president to change the course of history. When Andrew Johnson took over after Abraham Lincoln’s assassination in 1865, he scaled back the ambitious reconstruction project Lincoln and his right-hand man, Edwin Stanton, had prepared. Instead of focusing on rebuilding the war-ravaged country, Johnson and Republicans went to legislative war with each other.

Republicans passed the Tenure of Office Act, which restricted the president’s power to remove certain officeholders without the Senate’s approval. Johnson believed the law was unconstitutional and decided to challenge it by replacing Stanton. Republicans in the House responded by impeaching Johnson, passing 11 articles in February 1868. The Senate, however, fell short of convicting and removing Johnson by just one vote.

After William McKinley was assassinated in 1901, Teddy Roosevelt took over and turned the tides on big business. McKinley was famously backed by wealthy industrialists (who some described as robber barons). His campaign manager, Mark Hanna, once said, “There are two things that are important in politics. The first is money, and I can’t remember what the second one is.” The trusts that funded McKinley’s campaign were the ones T.R. busted when he became president.

When Franklin Roosevelt’s health was failing during the 1944 election, and war was still raging in Europe and the Pacific, administration and party bosses wanted a vice president who would be widely accepted as president and keep the country united. They replaced the progressive Henry A. Wallace with the more moderate Harry Truman. Truman’s steady hand calmed anxious Americans, oversaw the end of the war, and played a vital role in establishing the United Nations to prevent future conflicts.

Lyndon Johnson’s bet paid off when Kennedy was assassinated in 1963, helping him reach the pinnacle of power he craved. He continued Kennedy’s civil rights policies by signing the Civil Rights Act of 1964. He also launched the Great Society, expanded federal government and welfare programs, and escalated the Vietnam War with broad military powers granted by the Gulf of Tonkin Resolution. In 1965, while Johnson was still president, Congress passed the 25th Amendment, codifying the “Tyler precedent” and empowering the vice president and a majority of Cabinet officials to remove the president if he is unfit for office.

It’s not surprising Trump isn’t aware how important the vice president is. Like LBJ, Trump is a master of power but often lacks the historical knowledge necessary to wield it effectively for anyone but himself and his interests. Trump has now surpassed Joe Biden as the oldest person ever nominated for president. He’s been impeached twice. He’s had COVID and survived an assassination attempt. The odds that he'll finish his term are stacked against him.

Vance is keenly aware of the power of the vice presidency. He has argued that Harris and the Cabinet should invoke the 25th Amendment to remove Biden. “If Joe Biden can’t run for president, he can’t serve as president and, if they want to take him down because he’s mentally incapable of serving, invoke the 25th Amendment,” Vance said last month.

Because of the precedent set by John Tyler and others, and concerns about Trump's health and competency, Vance would wield significant power as vice president. If he invoked the 25th Amendment should Trump become ill or unfit for office, it would likely divide the country even further.

The key question then becomes: Who is JD Vance, and what does he really stand for? Is he a moderate like Truman, who balanced competing factions, or a radical like Johnson and Tyler, who imposed his policies on the American public through sheer determination and manipulation of power? We need a better sense of the man in the next three months.