The debate at the Longwood University in Virginia is a proxy war for the top of the ticket, and so the pressure is on.

As well as avoiding mistakes and introducing themselves to the American public, Democrat Tim Kaine and Republican Mike Pence have to defend their candidates as the increasingly bitter presidential race intensifies.

That might be easier said than done.

Indiana governor Mike Pence is a deeply religious, traditional conservative - in many ways, he is everything Trump is not. 

But the measured, cautious man might have to stand by a long list of Trump-related controversies.

There's Trump's decision to bring up Bill Clinton's infidelities, new questions over his charitable foundation, accusations of sexism, and the revelation he may not have paid federal taxes for almost two decades.

Former Virginia governor and senator Tim Kaine certainly has plenty of ammunition, but the Spanish-speaking, self-proclaimed "boring" VP pick has acknowledged that he will be spending most of the debate talking about Hillary Clinton.

He will have to fend off accusations that she is an untrustworthy member of an elite political establishment.

Both men are experienced, skilled politicians with a reputation for decency.

It is unclear whether either will reflect the increasingly barbed attacks of their bosses, who will meet again at the second presidential debate in St Louis on Sunday.

The first and only VP showdown, which kicks off at 2am Irish time, will be streamed online on Facebook, Twitter and YouTube. 

News sites including CNN, Buzzfeed, CBS News, C-Span and the Wall Street Journal will also be broadcasting the debate live.