Legend has it that before he accepted Jack Kennedy’s offer to be his vice-presidential running mate, made at the Democratic National Convention in July 1960 in Los Angeles’ Biltmore Hotel, senator Lyndon B Johnson of Texas calculated the odds on a vice-president stepping into the presidency. The odds were quite striking as, of 34 presidents seven had been vice-presidents who had been required to step up on the death of the leader either through ill-health or assassination.

At the time, Johnson occupied the powerful Senate majority leader position and some were curious as to why he would seek an office once described by a fellow Texan who had been Franklin Roosevelt’s first vice-president, “Cactus Jack” Garner, as not being worth a bucket of warm spit. But LBJ did have an appreciation of American history and he understood the vice-presidency could be a stepping stone into the White House. So, tragically, it proved to be.

The presumptive Democratic nominee for the 2020 US presidential race is former vice-president Joe Biden, who faces a serious test of his judgment next week, with extensive vetting completed, to choose a running mate. Already, he has made it clear this will be a woman and this has been welcomed.

The vice-presidential nominee has to be seen as a potential president, able to assume office in the event Biden actually is a one-term president only, or through the mechanism of the 25th Amendment to the US Constitution. So, a serious capacity to lead and to govern needs to be in evidence.

But it’s worth looking at the core qualities that make the difference between a successful VP candidate and potential president and those who bring little or nothing to the table. There is little doubt, for example, as Robert A. Caro records in his landmark fourth volume of The Years of Lyndon Johnson, The Passage of Power, that Johnson’s presence on the ticket in 1960 delivered Texas.

So too did Spiro T Agnew, governor of Maryland, play an invaluable role in Richard Nixon’s “Southern Strategy” in his successful presidential campaign of 1968. So Biden needs to look at his running mate’s electoral appeal, especially within African-American constituencies, which guaranteed him the latter presidential primaries. A number of very able candidates is on offer, ranging from Senator Kamala Harris of California, Senator Tammy Baldwin of Wisconsin, through Senator Tammy Duckworth of Illinois, to Governor Gretchen Whitmer of Michigan or Governor Michelle Lujan Grisham of New Mexico. Former UN ambassador Susan Rice is not only on a shortlist but is actively campaigning. Congresswomen Karen Bass of California and Val Demings of Florida are also in focus, along with Atlanta Mayor Keisha Lance Bottoms and her fellow Georgian, Stacey Abrams, among others.

But in this presidential contest, there is an overriding factor that is seldom present. Biden will be 78 if elected and thereby become the oldest president in the history of the republic. So the vice-presidential nominee has to be seen as a potential president, able to assume office in the event Biden actually is a one-term president only, or through the mechanism of the 25th Amendment to the US Constitution. So, a serious capacity to lead and to govern needs to be in evidence.

President Woodrow Wilson was incapacitated by a stroke in October 1919 while campaigning for American membership of the League of Nations. The last year of his presidency was characterised by an incapacity to govern. His vice-president, Thomas Marshall, should have assumed the presidency, but first lady Edith Wilson effectively transmitted decisions on her stricken husband’s behalf. So a vice-president needs to have the talent for leadership in a crisis and so add to the depth and appeal of a presidential campaign.

Certainly, Biden himself broadened the appeal of the Democratic ticket in 2008, given his considerable experience in foreign policy. Likewise, Dick Cheney lent gravitas to George W Bush’s Republican campaign in 2000.

The reverse is also true. Vice-presidential candidates can damage a presidential campaign when ineptitude or inexperience emerge. The classic case is senator John McCain’s decision to select outlier governor Sarah Palin of Alaska as his Republican VP running mate in 2008.

Palin was clearly out of her depth, which became obvious early on in interviews as in the infamous “Russia interview” with Katie Couric on CBS Evening News, which was both painful and revealing.

Palin was clearly out of her depth, which became obvious early on in interviews as in the infamous “Russia interview” with Katie Couric on CBS Evening News, which was both painful and revealing.

Constitutionally, the vice-president occupies no formal role in the US government as Nixon discovered as Dwight D Eisenhower’s. Technically, the vice-president presides over the Senate and has a casting vote. But he or she occupies no executive office, although traditionally the American space program has come within the VP’s remit.

But Biden’s decision is of great consequence. In 1864, fearing defeat that November, Abraham Lincoln dropped his abolitionist vice-president, Hannibal Hamlin, from Maine and embraced the military governor of Tennessee, a war Democrat, Andrew Johnson, to become his running mate. Following Lincoln’s assassination, the drunkard Johnson became president, narrowly escaping impeachment and proceeding to dismantle enlightened Republican plans for reconstruction of the defeated South. Johnson was the worst president in American history and the legacy of his bigoted decisions regarding African-Americans is still with us today.

Fortunately, the vice-president is no longer kept in the dark about critical issues of American national security. Harry Truman only learned of the Manhattan Project and the building of the atom bomb after FDR died in April 1945. Of more recent times, certainly since George HW Bush filled the vice-presidency in the Ronald Reagan administration, the VP is usually in the loop. Bush insisted on a weekly private lunch with Reagan and this has become the norm for administrations on both sides of the aisle.

American political parties are always geographical entities, given the vastness of the US and the diversity of its regions, its electorates and its interests. This means that a vice-president from the northeast for the Biden campaign is unlikely, so Senator Elizabeth Warren has an additional hurdle. Moreover, Biden is seen by most Democrats as being a centrist so a running mate who stands slightly further to the left is acceptable. The desired impact here is to mobilise the vote in a voluntary system where the quirks were most in evidence in 2016, when Hillary Clinton won the popular vote by some three million but lost narrowly in the Electoral College.

Biden’s decision could prove the difference in a close contest between making history or merely being a historical footnote.