The electorate’s huge yes vote for same-sex marriage shows that a conservative Liberal candidate might not be certain to win a byelection

With Malcolm Turnbull planning to resign from parliament imminently, the focus has returned to his seat of Wentworth. This blue-ribbon seat in Sydney’s eastern suburbs has been a crown jewel of the Liberal party and its predecessors since federation. But there are some local political elements that might make the Liberal party’s hold on the seat a bit shaky in an upcoming byelection.

The seat has existed since federation and has always been held by conservative parties. The Liberal party has held the seat uninterrupted since it was founded in 1944.

Numerous senior Liberals have held the seat. Successive members for Wentworth from the 1940s until the 1980s served as senior cabinet ministers. Liberal leader John Hewson represented Wentworth from 1987 until 1995.

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Malcolm Turnbull won Wentworth in 2004 after a fierce preselection battle which saw a flood of new members joining the local party as Turnbull blasted out his predecessor Peter King.

The seat had gradually become more marginal from the 1980s until the early 2000s, and was considered seriously marginal at the 2007 election, when a redistribution reduced Turnbull’s margin to 2.5%.

Turnbull survived the 2007 election, gaining a small swing in the face of Liberal losses across the country, and then gained large swings in 2010 and 2013 to make Wentworth one of the safest Liberal seats in the country. It is currently considered the eighth-safest coalition seat in Australia with a margin of 17.7%.

This recent history suggests that, while the Liberal party would be favourites to hold on, they have no guarantee of success without Turnbull as candidate.

The electorate stretches from Elizabeth Bay and Paddington close to the city centre, up to Vaucluse, and down the coast to Clovelly. It includes some of the wealthiest suburbs in Australia, covering prime beachside and harbourside real estate. Yet these suburbs are not consistently pro-Liberal.

Liberal voters in Wentworth tend to be more socially progressive than those in other parts of the country. More than 80% of voters supported marriage equality in last year’s postal survey, which was the fourth-highest yes vote in Australia. The Liberal party’s lurch to the right, and rejection of Malcolm Turnbull, could make things more difficult in Wentworth.

The Liberal vote is particularly strong in the harbourside suburbs from Double Bay to Watsons Bay, with more than 70% of the two-party-preferred vote in every booth in this area, and the vote peaking at 91% in Vaucluse.

Yet the Liberal vote, while still enough to win a majority, is much weaker in the beachside suburbs in the south-east of the electorate, with just 60.2% of the two-party-preferred vote in the booths within the Waverley and Randwick council areas (such as Bondi, Clovelly and Bronte).

The Liberal vote is also weaker at the western end of the seat, in areas such as Paddington and Elizabeth Bay.

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These areas (Waverley and Paddington) voted for Labor at the 2007 election, although a redistribution has removed Woolloomooloo and Darlinghurst, which voted strongly for Labor in 2007. These areas have also voted for non-Labor candidates at a state level in the recent past. The state seat of Sydney, held by independent Alex Greenwich, overlaps with the western end of Wentworth, while the south of the seat is contained in the state seat of Coogee, which was held by Labor until 2011.

You would expect a swing against the Liberals if Turnbull were to depart. Both Labor and the Greens (who are quite strong in Wentworth) would hope to increase their vote. A strong independent could also have the potential to draw votes away from the Liberal party and could win with Labor and Greens preferences, but it’s not clear who that person would be. Alex Greenwich could do well, but only a small part of his electorate overlaps with Wentworth, and he has announced he won’t run.

In the absence of a credible independent, it seems unlikely that Labor or the Greens would win a Wentworth byelection, but they could make life difficult for Scott Morrison’s Liberal party.