Rishi Sunak's election call will have caught some by surprise - but victory will be a tall order

The prime minister has surprised many, including within his own party, with his decision to hold a general election on 4 July. There are few, if any indications, that he will lead his party to re-election.

His gamble must surely be that during the coming weeks, when the electorate is forced to choose between himself or Sir Keir Starmer, that enough will side with the devil you know rather than plump for change.

We don't know what finally persuaded him to go now rather than wait for the autumn.

But it is significant that in his article in The Times after the local elections, he cited the Sky News projection of a House of Commons that pointed towards a hung parliament.

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The estimate put the Conservatives on 26% with Labour nine points ahead on 35%, a much lower level of support than shown in national opinion polls.

Naturally, the prime minister chose to be as optimistic about this projection as he possibly could be. He ignored the fact that the Conservatives were on course to lose 130 parliamentary seats after posting one of its worst ever local election performances.

He chose to ignore the fact that Labour was poised to overtake the Conservatives as the largest party, instead preferring to emphasise that there was all to play for at a future general election.

There certainly were aspects of the local elections that are at odds with national polling which currently show a 20 point or so Labour lead over the Conservatives. Labour failed to make significant gains of council seats from the Conservatives, sharing the limelight with Liberal Democrats, Greens and, a variety of Independents.

In some parts of the country with large Muslim populations, Labour's vote haemorrhaged in favour of candidates standing specifically on the issues of Labour's chosen stance on the situation in Gaza.

The nine-point gap in the estimated nationwide vote between Conservative and Labour compares unfavourably with that which existed between the Major government and the Blair onslaught unleashed in the mid-1990s prior to Labour's eventual general election landslide in 1997.

Labour believes that this reading of the local elections underestimates its real electoral potential. The Sky News projection made it crystal clear that there was no adjustment being made for the position in Scotland where Labour is well-placed to exploit weaknesses with the SNP.

Recent polling shows the two parties neck and neck, a position that will lead to significant gains for Labour, thereby easing its path towards a Commons majority.

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The abdication of traditional Labour voters in places like Blackburn, Oldham, and Pendle seen at the local elections is not regarded as something that will continue into a general election, thereby undermining Labour's request for an electoral mandate.

That assumption is certain to be examined closely over the coming weeks.

On the same day as those local elections, Labour achieved yet another parliamentary by-election victory with a swing of over 20-points in the Blackpool South constituency. Starmer's party has performed better than Blair's did on this metric.

Then there is the issue of tactical voting, something which played out strongly before the 1997 general election and will be the subject of much speculation now.

The Conservative brand has become severely tainted, but it remains to be seen whether voter dislike of Sunak's government extends to voters actively switching allegiance, sacrificing their normal party preference for strategic punishment of Conservative candidates.

Most of these voters will be voting in new parliamentary constituencies, boundaries re-drawn since the 2019 general election. These changes gave a small advantage to the Conservatives, effectively increasing its 80-seat majority to a 94-seat cushion.

The set of swing measures that gives context to the July election are worth setting out in detail. A national swing of just over four points from Conservative to Labour sees the government lose its overall majority.

Labour requires a swing of over eight points to overtake the Conservatives as the largest party in a hung parliament. This is several points larger than Margaret Thatcher achieved in 1979.

Labour goes into the election needing to win an additional 125 seats and a national uniform swing of 12.7 percentage points. A working majority of say 30 seats means a 13.8-point swing, substantially more than the 10-point swing Blair achieved in 1997.

Sunak has certainly gambled, going early rather than waiting for better news in the autumn.

It is a fact that no party has ever succeeded in recovering from a position where the Conservatives find themselves in the national polling. We won't have long to find out whether his gamble pays off.