Former President Donald Trump named JD Vance as his running mate on Monday during the first day of the Republican National Convention, thrusting the junior U.S. senator into a role that historically gets little attention once the party’s convention ends but could receive heightened attention this year.

Vance, 39, is one of the youngest major-party vice presidential choices in history — and a symbol of generational change in an election dominated by questions about age.

Never before has an 81-year-old president faced a 78-year-old challenger. Not to mention that polls show neither President Joe Biden, the older candidate, or Trump gets very favorable ratings in polls.

Saturday’s attempted assassination of Trump also brought into jarring clarity the ways in which a running mate or vice president can suddenly matter.

Since Biden and Trump are the oldest presidential candidates in American history, and Trump has been impeached twice and is a convicted felon, “it’s plausible that voters may weigh the vice-presidential choices somewhat more heavily this election,” said Joel Goldstein, professor emeritus of law at St. Louis University and an expert on vice presidential politics.

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Republicans are well aware of that possibility. Party loyalists have been pounding away at the idea that Vice President Kamala Harris is likely to succeed Biden before a second term is finished.

One of their favorite refrains is that Harris, a former California attorney general and U.S. senator, was picked for the Biden ticket four years ago because she was incompetent and therefore would not upstage her boss.

Conservative commentators have repeated that idea, and Trump emphasized it last week when he spoke in Miami.

“If Joe had picked someone even halfway competent they would have bounced him from office years ago, but they can’t because she’s gotta be their second choice,” he said.

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Does a VP pick have any impact?

Known for his book “Hillbilly Elegy,” which described his upbringing in a family that saw its fortunes decline, Vance was once a big critic of Trump, but in recent years has been a fierce loyalist. He won his Senate seat in 2022.

Democrats immediately attacked Trump’s running mate.

“Vance has championed and enabled Trump’s worst policies for years,” Democratic National Committee Chairman Jaime Harrison said in a statement, “from a national abortion ban, to whitewashing January 6, to railing against Social Security and Medicare.”

Rarely has there been solid evidence that even controversial vice presidential choices make a difference in an election’s outcome.

“By a day or two after the announcement, no one cares,” said Dennis Goldford, professor emeritus of political science at Drake University in Iowa.

Even when there is turmoil, there’s scant evidence it had an impact. In 1956, President Dwight Eisenhower was regarded as unusually old to run for a second term (he was 66). He had suffered a serious heart attack the previous year and had intestinal surgery in June.

His running mate was Richard Nixon, the vice president then known as an outspoken, communist-hunting, polarizing figure.

Eisenhower won in a landslide.

In 1988, Democratic vice presidential candidate Lloyd Bentsen had a sterling resume: Veteran senator from Texas, moderate, tough debater.

Republican opponent Dan Quayle said in their debate he had as much experience as John F. Kennedy did before becoming president. Quayle had been in Congress 12 years. Kennedy had 14 years in Congress before being elected president.

A steely Bentsen coolly shot back at Quayle, “I served with Jack Kennedy. I knew Jack Kennedy. Jack Kennedy was a friend of mine. Senator, you’re no Jack Kennedy..”

The line became iconic, but Quayle’s ticket, headed by George H. W.Bush, easily won the election.

Two ways a VP makes a difference

There have been two paths to making a vice presidential candidacy meaningful.

One is an ability to bring in voters who may be wary of the presidential nominee. Lyndon Johnson, then a Texas senator, is credited with delivering his state for Kennedy in 1960, a crucial pickup.

“Many of our elections are close, and the electoral college system means that a vp choice who affects a small, but decisive, number of votes in a swing state or two might impact the outcome,” said Goldstein.

The other route to influence has been more subtle — it involves sending a message to voters about the president.

Delaware was probably not crucial to Barack Obama’s victory strategy in 2008, but picking Washington veteran Joe Biden, then a long-serving U.S. senator from Delaware, signaled that relative newcomer Obama wanted an insider on his team. Ditto Jimmy Carter’s 1976 choice of then-Minnesota Sen. Walter Mondale.

“I think George W. Bush’s choice of Dick Cheney in 2000 may have helped,” Goldstein said. “That choice suggested that Bush would surround himself with seasoned hands.”

Cheney had been President Gerald Ford’s chief of staff, House Republican leader and George H. W. Bush’s defense secretary.

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Does Trump’s VP pick matter?

The biggest unknown this year is likely to be how much the Republican choice matters.

Harris is already well-known and opinions of her are largely set, pollsters say. In the July 1-2 Reuters/Ipsos poll, Harris was viewed favorably by 40% of registered voters and unfavorably by 57%. But she did win the office in 2020 on Biden’s ticket.

A lot of the opinion about her is a reflection of how people feel about Biden. “The very nature of the vice presidency presents a challenge in terms of connections with the public, and when you add in the intersection of Harris’s gender identity, multiracial background and California roots it seems a barrier to broad public acceptance is created,” said Christopher Borick, director of the Muhlenberg College Institute of Public Opinion.

“These factors, along with her personal style, don’t easily mesh with many Americans’ comfort zone, and Harris ultimately emerges with a classic likability problem,” he said.

There are no credible national polls to measure the impact of a Republican vice presidential pick, and the general view is that Trump is such a strong, polarizing personality that it would have little impact.

He’s reportedly not considering anyone from a swing state. Florida, Sen. Marco Rubio’s state last gave its electoral votes to a Democrat in 2012, and Trump won the state handily in 2020.

North Dakota, home of Gov. Doug Burgum, hasn’t voted Democratic for president since 1964.

Ohio, where J. D. Vance is the junior senator, for decades was the ultimate presidential bellwether, but Trump won it easily in 2016 and 2020.

Experts keep adding that asterisk to all that conventional wisdom about vice presidential candidates, because Biden would be 86 at the end of his term and Trump would be 81.

“As a cold, hard fact of life, that makes the next vice presidency potentially more important than it usually is,” said Goldford.

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