Tim Kaine, Mike Pence little known as they head into VP debate

Tonight's vice-presidential debate between Mike Pence and Tim Kaine will feature two of modern history's least-known vice-presidential candidates, vying to be next in line to two of the oldest U.S. presidential candidates ever to stand for the top job. The stakes are high.

But the debate is unlikely to have a major impact on the comparatively epic tilt between Hillary Clinton and Donald Trump.

• A 'heartbeat away' from the Oval Office: How Tim Kaine and Mike Pence stack-up
• Hillary Clinton's support grows in post-debate polls
• Presidential Poll Tracker: Check out the latest projections

Mike Pence, the Republican candidate, and Tim Kaine, for the Democrats, are unknown to many voters. According to Pollster's averages, one-third of Americans have no opinion of Pence, the Governor of Indiana. Kaine, a senator and former governor from Virginia, is a mystery to 40 per cent of voters.

And neither vice-presidential candidate elicits strong opinions either way. About 36 per cent of voters hold a favourable view of Pence, while 31 per cent hold an unfavourable view. For Kaine, the split is 30-30.

So the vice-presidential debate will provide an opportunity for these two men to introduce themselves to America. But this will also make the debate somewhat unusual, as the running mates are often much better known.

At the moment of 2012's vice-presidential debate, just 11 per cent of Americans had no opinion of Democratic VP Joe Biden, while only 16 per cent had no view on Mitt Romney's Republican running mate, Paul Ryan.

About a fifth of voters had no opinion on Biden or Sarah Palin, John McCain's vice-presidential pick, at the time of the 2008 debate.

Similarly small proportions of Americans had no opinion of John Edwards or Dick Cheney, the two vice-presidential candidates in 2004. Even the 2000 vice-presidential debate between Cheney and Joe Lieberman still occurred at a time when less than a third of Americans had no views about either candidate.

Low-impact debating

It is difficult to know whether vice-presidential debates have had any significant impact on the polls in recent campaigns. This is largely due to these confrontations taking place between the first and second presidential debates, which are far more likely to have a big impact on voting intentions.

In the last three election campaigns, only a handful of polls had been conducted between the first presidential and vice-presidential debates, making it impossible to tease out any role the vice-presidential debates might have played in any shift. 

This will be the case again in 2016, as the next face-off between Trump and Clinton will take place on Sunday.

However, that doesn't reduce the importance of the debate. Combined, Trump and Clinton are the two oldest candidates to ever run for office. The possibility that whoever becomes vice-president will become president before the 2020 election is higher than it is in most campaigns.

So will Americans tune in?

Ratings bonanza or bust?

A record 84 million people watched last week's debate between Trump and Clinton. It is very unlikely that the vice-presidential debate tonight will have as large an audience.

Nevertheless, the audience for vice-presidential debates in the past has been respectable. In 2012, 51 million Americans tuned in to watch the Biden/Ryan debate.

Since 1976 the vice-presidential debate has had an audience of between 62 to 88 per cent of the first presidential debate in all but one case. That means between 52 and 74 million Americans could watch the Pence/Kaine debate tonight.

The higher-end of that range, however, would make the debate the most watched bout between the running mates in America's history. The current record-holder is the debate between Biden and Palin in 2008, when the audience of 70 million was significantly larger than the first presidential debate in that campaign, between Barack Obama and McCain. It seems unlikely that Pence and Kaine will draw as much interest.

All politics is local

The best way for a vice-presidential candidate to have an impact on his or her running mate's campaign is at the local level. On average, vice-presidential candidates have boosted their campaign's chances in their home state by a little more than two points. 

Neither Indiana nor Virginia, however, are particularly close states. The Presidential Poll Tracker currently gives Trump a 16-point lead in Indiana, while Clinton is ahead by six points in Virginia (and consistently so). Both Trump and Clinton would likely be winning in these states whether or not they had chosen Pence and Kaine.

However, recent polling suggests the presidential campaign has had an impact on Pence and Kaine's popularity at home.

A recent Morning Consult poll shows that Pence's approval rating in Indiana is split down the middle with 45 per cent approval to 45 per cent disapproval. That is a net decrease in his approval rating by nine points since earlier this year, making him one of the least popular governors in the United States.

Kaine, on the other hand, has seen his numbers improve in Virginia. Polling by PPP shows his approval rating up five percentage points since June, to 50 per cent approval against 37 per cent disapproval in his home state.

The vice-presidential debate tonight will primarily be about the names at the top of the ticket. But to the people who know the running mates best, it appears that having to defend and promote Donald Trump and Hillary Clinton has been hurtful to one and helpful to the other.

If that plays out on the debate stage, Tim Kaine may have the better night. At a minimum, all Kaine needs to do is ensure he doesn't cause a distraction that might stall Clinton's forward momentum. Nothing Mike Pence can do, on the other hand, is likely to single-handedly turn Trump's campaign around.