On Monday, Donald Trump picked Ohio Senator JD Vance as his Republican running mate in the United States presidential election, following weeks of speculation over who the real estate mogul and ex-president would choose.

On the opposing ticket, meanwhile, is Kamala Harris, running with Democrat Joe Biden. Harris is the incumbent VP who ran with Biden in 2020, when the California senator became the first Black woman and the first Asian American to compete on a major party’s presidential ticket.

Since the Vance announcement, analysts have pored over possible reasons why Trump might have selected the former venture capitalist and author who until a few years ago was a trenchant critic of the ex-president.

But does the choice of VP actually boost a presidential candidate’s chances of winning the election? Al Jazeera breaks down decades of election results, polls and analysis to find out.

Do presidents perform better in the VP’s home state?

That’s often a central consideration that political insiders cite – the hope that the VP pick might help the ticket win his or her state.

However, researchers who have studied election results over decades say there is little to suggest that this calculation actually helps.

“It’s very rare that we find that a running mate would deliver a particular home state,” Kyle Kopko, an adjunct professor of political science at Elizabethtown College in Pennsylvania told Al Jazeera.

Kopko has extensively researched and written about the electoral effect of running mates with Christopher Devine, political science professor at the University of Dayton.

During his research, Kopko has found that VPs can mobilise more votes for presidential candidates most frequently if they come from a small state and have a great deal of political experience. This does not apply to JD Vance.

An example of a running mate like this was Biden himself, when he was former President Barack Obama’s VP in the 2008 and 2012 elections, Kopko said.

Biden comes from Delaware, a small state with only three counties. He had “a tremendous amount of political experience serving in the Senate from Delaware”, said Kopko.

“But Delaware was already a pretty consistent Democratic state in the Electoral College.”

How have presidents historically fared in their VP’s state?

While presidential hopefuls typically do win in their running mates’ home states, they have in recent decades almost always chosen VP picks from states they were anyway expected to win — and not from swing states.

When they have chosen VP candidates from states in the balance, the results have been mixed – at best.

Consider 1960, when Democrat John F Kennedy, won Texas, the home state of his running mate Lyndon B Johnson, with 50.5 percent votes.

Both Kennedy and Johnson said that if it weren’t for Johnson, Kennedy would not have made gains in the South. Kopko said that this is where the myth of the VP’s home-state advantage comes from.

The Democrats, who had traditionally dominated Texas politics, had lost in the state in 1952 and 1956 – and so could use a boost. However, Kopko’s analysis of survey data from that election shows that Johnson was actually unpopular among the state’s voters and might have hurt Kennedy in Texas. The race in the state was close – Kennedy defeated Richard Nixon by 2 percentage points.

In 1992 and 1996, Democrat Bill Clinton won in Tennessee, the state of his running mate Al Gore. It was the first time that Democrats had won Tennessee since 1964. But was Al Gore responsible? In 2000, when Gore was his party’s presidential nominee, he lost in Tennessee to George W Bush.

If Gore’s 2000 loss in Tennessee showed that presidential candidates aren’t guaranteed wins in their state, that also holds true for VP nominees.

In the 1968 election, while Republican Richard Nixon won the presidency comfortably, Democrat challenger Hubert Humphrey won in Maryland, the home state of Nixon’s VP Spiro Agnew.

What about recent years?

• In 2020, Biden won in California, Harris’s state, with 63.5 percent of the votes. However, since 1992, a Democratic candidate has always won in California. Indiana Governor Mike Pence was Trump’s VP and Trump won in the state with 57 percent of the votes. Since 1968, Republican candidates have won in Indiana in every election except 2008, when Democrat Obama won.
• In 2016, Trump, who was running with Pence, won in Indiana with 57.2 percent of the votes. Hillary Clinton was running with Virginia Senator Tim Kaine for the Democratic Party. Clinton won Virginia with 50.2 percent of votes.
• In 2012, Obama ran with Biden as his VP and swept Delaware with 58.6 percent of votes. Wisconsin representative Paul Ryan was Republican challenger Mitt Romney’s running mate. Romney did not win in Wisconsin, where Obama grabbed 52.8 percent of the vote.
• In 2008, Obama won in Delaware with 62 percent of the vote. Republican challenger, John McCain, picked former Alaska Governor Sarah Palin to run with him. McCain won in Alaska with 50 percent of the vote.

What do past polls tell us?

Even if presidential candidates perform marginally better in their running mate’s home state, as some studies suggest, their overall national popularity seems largely unaffected.

In the last election, Biden announced Harris as his VP on August 11, 2020.

Based on the voting analysis platform FiveThirtyEight’s average of 2020 presidential election polls, Harris did not have a significant impact on Biden’s popularity among voters.

In late February 2020, Biden and Trump were running close in polls, with Biden only 3.8 percentage points ahead of his Republican competitor. This gap tripled to 9.5 percentage points by late June 2020, before Harris was announced as Biden’s VP.

On August 3, 2020, Biden was 8.2 percentage points ahead, at 50.5 percent against Trump’s 42.3 percent. By August 24, Biden’s poll performance only saw a meagre boost; he was at 51.4 percent on the polls.

For the 2016 election, Trump announced Mike Pence as his running mate on July 15, 2016, while Democratic competitor Clinton chose Kaine as her VP on July 22, 2016.

The competition between Trump and Hilary Clinton was close on June 9, 2016, with Clinton only 4 percentage points ahead, according to the FiveThirtyEight national poll average that year. The gap closed further to 3.5 by July 14, 2016.

By July 30, 2016, after both VP picks had been announced, Clinton and Trump were neck and neck, polling almost identically. However, the gap grew and August and September saw the biggest difference in percentage points between Clinton and Trump, peaking to 8.1 during this time.

Do VPs allow presidential candidates to do better with certain demographics?

When Biden picked Harris as his running mate, analysts predicted it would boost support for Biden among Black voters.

A poll conducted by Northwestern University’s Center for the Study of Diversity and Democracy during the summer of 2020 found 57 percent of African Americans responded they would be more enthusiastic about voting for Biden if he chose an African American woman as his VP.

After the election, a CBS exit poll showed 90 percent of Black female voters had supported Biden. However, Black women made up only 9 percent of the exit poll’s sample comprising 15,285 respondents.

Kopko said his analysis found little evidence of VPs improving votes among specific voter groups.

For instance, he said, there was little statistical evidence proving that Geraldine Ferraro, the running mate of Democratic candidate Walter Mondale in 1984, or Sarah Palin, John McCain’s 2008 VP pick, were able to mobilise women voters for their tickets, despite both being popular among female voters in opinion polls.

In fact, a Pew Research Center report breaking down the 2020 election results showed that Trump made inroads with women voters, gaining 44 percent of the vote share from women, compared with 2016 when it was 39 percent. This, in a year, when Harris was on the opposing ticket, and Trump had Pence as his running mate.

What other factors do presidential candidates consider?

If presidential candidates aren’t choosing running mates on their ability to turn swing states or because they can substantially attract demographics that wouldn’t otherwise vote for them, what are the other factors at play?

Kopko said some presidents pick a VP who aligns with their politics to reinforce their policy agenda to voters. He said that while it is difficult to determine Trump’s overall motivation behind picking Vance, he speculated Trump picked Vance because he would be easier to work with if Trump wins the election because their policy priorities overlap.

There could be another reason, too. In this third presidential race now, Trump has come up against a range of Republicans who have challenged him in 2016, 2020 or 2024, before – in most cases – falling in line and kissing the ring.

Vance, while a former critic of Trump’s, has never contested against him electorally.

“JD Vance wasn’t running for president. He wasn’t attacking Trump along the campaign trail,” Kopko said.