Indiana Governor Mike Pence and Virginia Senator Tim Kaine will arrive at a debate stage today with three main assignments: defend their bosses from attack, try to land a few blows, and avoid any mistakes showing them unfit to be president.
The understudies for Donald Trump and Hillary Clinton, both veteran politicians with folksy dispositions, will be making the highest-profile and most-perilous appearances of their careers.
Yet even with the vice-presidential nominees seated alone at a table with moderator Elaine Quijano of CBS News, the focus of the 90-minute session is expected instead to be on the historically unpopular candidates at the top of the tickets and the mistakes they've made.
Pence could have the bigger lift, as the one and only vice-presidential debate convenes at 2pm NZT at Longwood University in Farmville, Virginia.
Trump is trying to recover from one of the worst weeks of his presidential campaign following a shaky debate performance, his comments disparaging a beauty pageant winner's weight and personal life, and a New York Times report that he may not have paid any federal taxes for almost two decades following a nearly US$1 billion business loss.
With polling showing a shift in momentum towards Clinton, Pence needs to avoid being an apologist for Trump and get the campaign back on offence, said Brett O'Donnell, a Republican strategist who helped prepare Alaska Governor Sarah Palin for her 2008 debate with Vice-President Joe Biden.
"A lot of people will be looking at this debate to see if Mike Pence can put the campaign back on the rails again," he said.
Any gains or losses could be short-lived. Clinton and Trump will convene for their second of three debates on Monday in St Louis.
Pence, who is known for message discipline, tends to deflect questions about the undisciplined Trump and pivot to the Republican standard-bearer's selling points as a status-quo disruptor in a political environment hostile to Washington insiders.
Kaine must use every opportunity to press Pence and keep him on the defensive about Trump's record and statements, while helping make Clinton's case that the billionaire lacks the temperament and qualifications to be president, said Tad Devine, a Democratic strategist who was campaign manager for then-Texas Senator Lloyd Bentsen when he was Democrat Michael Dukakis' running-mate in 1988.
Bentsen famously skewered Indiana Senator Dan Quayle in their 1988 debate with the line, "Senator, you're no Jack Kennedy," before he and Dukakis went on to lose big to Quayle and George Bush snr.
"The more he can dredge up that stuff and hit him on it, the way Hillary very effectively did in the first debate, the more points they'll score," said Devine, who more recently worked on Vermont Senator Bernie Sanders' presidential campaign against Clinton before Sanders endorsed the former Secretary of State.
Both Devine and O'Donnell said they expect Pence will try to turn quickly to economic and foreign policy issues and make the case for a different Republican approach than the past eight years under President Barack Obama.
"If he can stand there for 90 minutes and say that Trump represents change and here's how, and make a strong case for it, I think that's going to be a strong performance," Devine said.
Although there will be just a few hundred people seated inside the debate hall on the university campus, tens of millions more will be watching the nationally broadcast event.
Viewership is certain to be less than for the first debate between Clinton and Trump on September 27, an event that drew a record political audience of at least 84 million viewers, according to Nielsen.
An estimated 51.4 million people watched the 2012 vice-presidential debate between Biden and Republican Congressman Paul Ryan, about 18 million fewer than for the 2008 encounter between Biden and Palin, according to Nielsen.
Pence, 57, who had been seeking a second term as Indiana Governor after previously serving in the US House for 12 years, joined Trump's ticket in mid-July.
A former radio-talk show host who became a born-again Christian in college, Pence has championed limited government and social causes such as opposition to abortion that appealed to conservatives and evangelicals.
Kaine, 58, started his political career in the 1990s as a Councillor in Richmond, Virginia. He eventually became the city's Mayor and then Virginia's lieutenant governor and Governor. From 2009 to 2011, he was the chairman of the Democratic National Committee and he has been a US senator from Virginia since 2013.
The senator could press Pence to defend the religious-freedom law he signed last year that drew swift opposition from business executives and gay-rights groups as being discriminatory. Pence later sought and signed a measure that bars businesses from refusing to serve gays and lesbians on religious grounds to quell the furore.
Pence also has advocated for anti-abortion bills and measures seeking to defund Planned Parenthood during his time in Congress and as Governor.
Before Trump picked him as his running-mate, Pence faced a difficult re-election fight in a rematch of his 2012 race with Democrat John Gregg. If the Republican ticket loses this year, he could be well positioned to make a run for the Republican nomination in 2020.
Kaine could get hit in the debate for his acceptance of US$160,000 in gifts while he was lieutenant governor and Governor. Under then-lax ethics laws, he accepted favours that included gifts from companies with business before the state.
The gifts included a Caribbean holiday and a trip to watch George Mason University play in the NCAA basketball Final Four. The donations were legal, but that hasn't prevented Republicans from trying to use them to insinuate corruption.
As one of the most intense and polarised campaigns in modern American history enters its final five weeks, Pence and Kaine will both campaign tomorrow in Pennsylvania, a state critical to Trump's path to the White House.