To say Napier has been a Labour stronghold is an understatement - the 156-year-old electorate has been red for all but 17 years since the first Labour MP was elected in 1922.

It's had its share of interesting MPs - there was the one who changed parties halfway through his term (Bill Barnard, from Labour to Democratic Labour in 1940).

There were the four Labour MPs who together won 17 back-to-back elections, holding the seat for 62 years from 1954 to 2005 - with a 21-year-stint from Geoff Baybrooke.

Then there was the National MP who bucked the trends - becoming only the second National MP in the seat, which he held for three terms. Every other previous non-Labour MP lost the seat after their first term.

However, the electorate now appears to be at a crossroads, with growing support for National.

Labour attracted Napier voters for so long because of its high urban population, Massey University Associate Professor Christine Cheyne said.

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Voting could be affected by a number of factors, she said, including boundary changes, the socio-economic demographic of an area, and whether one industry had a particularly strong presence.

"Port cities tend to have quite a strong working class element, and so that will possibly be a part of it," Massey University associate professor Grant Duncan said.

Napier also has a higher than average Maori population, who are typically "strongly Labour", he said.

However, industrial and demographic shifts could have chipped away at Labour support.

"Twenty-five years ago the Labour Party had a 1000 member branch at Maraenui, these days it doesn't have a branch there," former Labour MP Russell Fairbrother said.

Napier is now home to more retirees, self-employed and business professionals, with a number of Maori now on the Maori roll.

Older populations tend to vote more conservatively, and fewer Maori on the general roll would "tip the balance a little bit more towards National", Mr Duncan said.

These changes could be why National support has increased - the last time Labour won the party vote was 2002, despite having two MPs in that period.

"You'd have to argue [Napier's] a strong National seat now with a Labour MP," Mr Tremain said.

In 2005 he brought an end to the 62-year hold Labour had on the electorate, beating incumbent Mr Fairbrother.

"It was a long run, and I lost what was considered to be a traditionally safe Labour seat. But if you toe the party lines as I tried to do, you bring in the party aspect," Mr Fairbrother said.

Mr Tremain credited his win with a change in voting composition, and a boundary change - with the "strong National supporters" of Taradale, and Greenmeadows becoming part of the electorate.

But while National has increased its hold, in 2014 its candidate lost by around 4000 votes to Labour's Stuart Nash. He is considered the front runner in this election, with his main opponent newcomer National candidate David Elliott.

This contrast could be down to candidates' local profile.

"If you've got a good public profile, people in Napier might vote for the person even if they don't support the party," Mr Fairbrother said.

Academics echoed this - "As a local identity, it is quite influential that clearly can override the factors that are often associated with voting for a particular party, particular economic factors," Ms Cheyne said.

Napier's political shift began in 2005, when National received 15,086 party votes, just ahead of Labour's 14,615.

In 2008 National's Mr Tremain won the seat for a second time, with 20,898 votes to Mr Fairbrother's 11,880. His party retained its lead, with 16,772 voting National, compared to 12,621 for Labour.

The margin between the two parties increased from 2011, when Labour received 9,921 party votes to National's 16,538. In that election Mr Tremain received 17,337 votes to Mr Nash's 13,636.

In 2014, Mr Nash stole the election, receiving 15,343 votes. National candidate Wayne Walford secured 11,493. However, National won the party vote, receiving 18,005 votes to Labour's 9466.