Donald Trump's VP pick of Mike Pence could boost his campaign, for now

In a normal U.S. presidential election, the choice of vice-presidential candidate usually hasonly a minimal impact on the race. Vice-presidential candidatescan help deliver their home state and provide a short-term boost to his or her running mate's campaign, but elections are rarely decided by who the "Veep" will be.

But this is not a normal election.

Donald Trump, Republican nominee for the U.S. presidency, announced Friday that Gov. Mike Pence of Indiana will be his pick for vice-president.

• Who is Mike Pence?
• How the 'last-ditch' plot to dump Trump may have just died

Hillary Clinton, the Democratic nominee, will make her pick before the party's convention in Philadelphia later this month.

Vice-presidential candidates are often chosen to fill a gap in the presidential candidate's resum, be it geographic, demographic, or political.With Pence as Trump'srunning mate, it would givehis ticketmuch-needed political experience, as well as a figure outside of the U.S. Northeast.

But vice-presidential picks normally do not have a significant impact on the race. The choice may help a presidential campaign at the margins, but the first rule of a vice-presidential candidate is often cited as "do no harm." Vice-presidentialpicksare at greater risk of dragging down a campaign than boosting it to new heights.

Clinton and Trump, however, are two candidates with the lowest favourability ratings in recent American electoralhistory. This may be a rare election in which the vice-presidential candidates can only help.

What kind of impact can we expect the picks to have on the state of the race?

'Veep' honeymoons fade fast

The examples of the last three U.S. elections suggest the impact a vice-presidential pick has on the polls can be significant but it doesn't last.

In 2004 and 2008, the vice-presidential candidacies of John Edwards and Joe Bidencoincided with aboost inthe Democratic campaigns of both John Kerry and Barack Obama, whowere only leading their Republican rivals by about a point when their picks were announced.

Within a week, Kerry was up on George W. Bush by three points and Obamawas ahead of John McCainby more than six.

On the Republican side, VP choices in 2008 and 2012 initially helped their losing campaigns. With the choice of Sarah Palin, McCain went from a four-point deficit to a lead of three points a little over a week later.

In 2012, Paul Ryan helped pushMitt Romney'strailingcampaign into a tie with Obama.

But after reaching a polling peak a week or two after the announcement of a vice-presidential running mate, in every case,these surging campaigns saw their numbers fall back down to Earth about as quickly as they had risen.

A large factor in this was the timing of the party conventions. Depending on when they were held, they tended to amplifythe impact of a VP pick or put an end to an opponent'srising numbers and sometimesboth.

Based on these examples, we might expect the addition of Pence to the Trump ticket, as well asthe Republican convention next week, to boost Trump'spolling numbers by three orfour points. But it mightendure no longer than the beginning of the Democratic convention on July 25.

Veeps can be a drag

While many vice-presidential candidacies have had only limited effects on the longer-term chances of their running mates,the example of Palin shows how a bad VP pick can be disastrous.

When Palin, then thelargely unknown governor of Alaska, was announced as McCain'spickon Aug. 29, 2008, the reaction was largely positive. AGallup survey taken a week later found that 53 per cent of Americans had a favourable view of Palin. Just 28 per cent had an unfavourable view of her.

But by the end of the campaign, her favourables had dropped to 42 per cent and her unfavourables had soared to 49 per cent. An ABC News survey taken just before election dayfound that just 17 per cent of respondents said Palin's candidacy made them more likely to vote for McCain, compared to46 per cent whosaid it made them less likely to vote Republican.

Biden, by comparison, was a net positive for Obama:twice as many people said it made them more likely to vote for himthanless likely. Most Americans, though, thoughtBidenmade no difference.

Palin, unfortunately for McCain, did.

Home-state advantage

If there is one thing that vice-presidents can do, it is help give theirparty a better chance of winning the Veep'shome state.

In presidential elections over the last 40 years, non-incumbent vice-presidential candidates have seen a bigger swing towards their respective partyin their home state than nationally.

In 2008, the Democrats increased their vote share by 8.5 points in Biden's home state of Delaware, compared to a gain of4.6 points nationally. In 2012, the Republicans gained 3.6 points in Ryan's home state of Wisconsin, compared to a gain of 1.5 points nationally.

Ryan, however, failed to carry Wisconsin for the Republicans.

But on average since 1976, a vice-presidential candidate has boosted his or herparty by 2.3 points in their home state, compared to the national swing.

That might make Pence an unusual choice for Trump. The Republicans are already leading in Indiana by a comfortable margin.Any boost Pence gives to the party in his home state wouldlikely be wasted.

But as part of Trump's strategy to carry states in the Midwest,the choice would makemore sense. Michigan and Ohio, two potential swing states, are just across the Indiana border.

Those states would help the Republican cause. But in this election, Trump may need more than a "do no harm" Veep.

• The Pollcast: Subscribe to ric Grenier's politics podcast
• Leader Meter: Track federal party leaders' approval ratings