On 20 July, Rishi Sunak could become the first prime minister since Harold Wilson in 1968 to lose three seats at by-elections on the same day.

The Conservative Party’s implosion over MP misconduct and whether Boris Johnson lied to parliament has presented election watchers with an intriguing set of contests.

Uxbridge and South Ruislip in west London, vacated by Mr Johnson himself, gives Labour a shot at a seat well within the swings (around eight points) they have already achieved in this parliament.

Selby and Ainsty in North Yorkshire, where Johnson ally Nigel Adams has stepped aside, requires an 18-point swing for a Labour win. This is beyond both the national 12-point swing the party needs for an overall majority at the next general election and the 16-point swing suggested by recent polls. Gaining Selby would also set a record for the size of majority overturned by Labour at a by-election.

Given stellar Liberal Democrat performances since 2021, Somerton and Frome should be easy pickings for the party David Cameron once obliterated from the South West. A swing of 15 points would topple yet another Tory seat in the south of England.

The results will give an insight into how voters view the government’s effort to tackle inflation, rising interest rates and NHS waiting lists and whether they think it’s time to give Sir Keir Starmer a go at solving them. But, as ever, by-elections develop their own character and local priorities can intervene.

So, what’s at play in Uxbridge, Selby and Somerton?

Uxbridge and South Ruislip

Established in 2010, the seat of Uxbridge has elected Conservatives since then. Boris Johnson won more than half the vote at each of the last three contests. It even bucked the London trend to back Brexit.

A suburban commuter town on the western edge of metropolitan London, it includes both a university and RAF Northolt. The area hasn’t experienced the same urban development as much of the capital, but the demographics have been moving in Labour’s favour.

The latest census suggests the population has become younger, more educated, and more diverse than a decade ago, all likely indicators of Labour support. That said, look a little closer and it’s an inconsistent picture.

Students dominate in the university areas around Uxbridge and Colham where more people live in rented homes. The working-class area of Yiewsley is the most Labour friendly. While South Ruislip is the main Tory territory. Here, you find older owner-occupiers and commuters. The rising Asian community also seems to have given the Tories a hearing.

These differences may be one reason why Uxbridge has been ‘sticky’ at election time. In 2019 Boris Johnson was defending the smallest majority of any prime minister since 1924, just over 5,000 votes. Despite Labour’s best efforts he increased that to over 7,000. It means Labour need an eight-point swing, just half that suggested by the national polls, to win the seat for the first time.

But, as ever at by-elections, it might not be that simple.

The most recent elections in the constituency were for Hillingdon Council in 2022 and the results showed little enthusiasm for Labour, despite a record Conservative defeat across the capital on the same day.

Labour won just one of the seven wards that sit entirely within the constituency – Yiewsley. To succeed on 20 July, they need to maximise their vote there and persuade the students of the Colham and Cowley ward to turn out.

But there’s a local factor dominating the contest that could render the national politics largely irrelevant.

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The Ultra Low Emissions Zone (ULEZ), a tax on cars which don’t meet certain emissions standards, is being extended by London’s Labour mayor to cover the area in August. Unlike inner London, this is a constituency where around four in five households have a car and one in three have two or more. The issue has prompted two of the extensive list of 17 candidates to change their names to include the phrases “Anti-ULEZ” and “No-ULEZ”.

The length of the ballot paper could also be a problem for the main parties. It gives voters plenty of options other than Labour, ranging from UKIP, which has done well here in the past, to Piers Corbyn (brother of Jeremy), and Laurence Fox. The Conservatives will hope voters read to the end as their candidate is listed last thanks to his place in the alphabet.

With a declaration not expected until after 3am, those watching Sky News will have plenty of time to consider the turnout. Invariably lower at by-elections than general elections, there’s no way of knowing why people do and don’t vote or who did and didn’t. However, we can estimate a reasonable figure.

Based on contests so far this parliament, we expect turnout to be around 27 points lower than in 2019 in all three constituencies. For Uxbridge that means something in the region of 40%.

Selby and Ainsty

A record-breaking result could come at the other end of England, in Selby and Ainsty. Nestled in the North Yorkshire countryside, this seat is a mix of rural villages and towns surrounded by churches and historical battle sites.

Almost everything about it says Tory heartland and since its creation in 2010 that’s how people here have voted. Nigel Adams’s decision to resign, because he was denied a peerage, means Selby will have a change of MP for the first time.

If Labour were to win, it would set a record. The highest majority the party has overturned at a by-election is 14,654 votes in Mid-Staffordshire more than 30 years ago.

But while the demographics here might not be trending in Labour’s favour, as more than a fifth of people are aged over 60, the issues are. Selby and Ainsty is in the top 40 seats in England and Wales for mortgage holders. 37% of households have a mortgage and rising interest rates might impact the vote.

The latest council elections in 2022 also provide Labour with hope. They finished just six points behind the Conservatives despite managing to win only four of the 15 wards within the constituency. Labour needs to persuade voters in Selby, Sherburn and Appleton who did not support them then to do so now.

Turnout could be key too. In Selby, we suggest one of around 45% would be in line with recent by-elections.

With the general election creeping closer, Sir Keir Starmer needs to show he can win votes directly from the Conservatives in places throughout England, not just those with Labour history. Selby provides the perfect opportunity.

The 18-point swing required for victory would be the best Labour has achieved this parliament. By-elections aren’t ideal predictors of general election performance but, if they do win Selby, no doubt Labour will remind us they need a 12-point national swing for a parliamentary majority.

That is more than the record swing achieved by Tony Blair in 1997. Intriguingly, the estimates drawn up for Selby’s boundaries for 2005, Blair’s last election, put Labour just four points shy of the Conservatives. So perhaps winning Selby should be within Labour’s reach if it is to win a majority at the next general election.

The pressure to pull off the big win is on Labour candidate Keir Mather. At just 25, if elected, he would be the youngest MP in the Commons – the so-called Baby of the House.

Somerton and Frome

In the West Country the Conservatives face a different challenger and a feeling of deja vu. Just a year ago the Liberal Democrats made by-election history overturning a record 24,239 Conservative majority in Tiverton and Honiton and now they’re back in Somerton and Frome.

Unlike Tiverton in Devon, this Somerset market town has substantial Lib Dem pedigree. For 18 years it was represented by the former Lib Dem minister David Heath until the Tories gained it in the 2015 post-coalition sweep of the South West.

David Warburton was the winner then and it is his departure after allegations of cocaine use, which he admits, and sexual misconduct, which he denies, that has triggered this contest. He had built a substantial majority of more than 19,000 votes but this seat has often been competitive.

The population is largely older than average and less exposed to interest rates, with more than 43% of households owning their home without a mortgage. But they have been trending away from the Tories.

The 2022 Somerset Council results were terrible for the Conservatives and the biggest falls were in the wards that make up Somerton and Frome. The Liberal Democrats were first in 10 of those 13 wards, taking 40% of the vote, while the Conservatives managed to win just one. Even the Greens managed two.

And you can’t say Sarah Dyke, the Liberal Democrat candidate, doesn’t know how to defeat a Conservative. At those 2022 council elections, she beat Hayward Burt, CCHQ’s resident expert on conquering Liberal Democrats, to take her seat in Blackmoor Vale.

Now, she requires a swing of 15 points to become the MP for Somerton, which looks pretty modest compared to other Lib Dem wins this parliament.

We expect a reasonable turnout to be in the region of 48%.

How to judge the result?

Rishi Sunak is unlikely to emerge from these by-elections unscathed.

On recent form, a loss in Somerton is expected and it will be further evidence the Conservatives could be fighting the next general election on two fronts.

Losing Uxbridge would be a blow to the Tories but no worse than other defeats in this parliament. Should Labour miss out, Sir Keir Starmer will have questions of his own to answer.

But attention will be elsewhere if Conservative rural Selby turns a record-breaking red. Labour could claim to be winning votes directly from the Tories even in their established heartlands.

Selby is the Conservatives’ 249th most vulnerable seat. If Labour wins that, who’s to say they can’t gain the 124 seats they need for a Westminster majority?

Watch live coverage of the by-election results on Sky News through the night from 11pm on Thursday 20 July.