July and December are always vicious months in the calendar at Westminster.

No matter when the monarch officiates at the official state opening of parliament, summer and winter are natural climaxes in the year when reckoning takes place.

In 2023, we have already had early casualties as parliament backed the reports of the standards and privileges committee. Boris Johnson has resigned as an MP and so have two colleagues, with more set to follow.

That is the overture to the drama which will unfold next week, when parliament is sitting for the last time before the long summer recess.

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On Wednesday, Rishi Sunak and Sir Keir Starmer are actually billed to turn up for the last PMQs until September, in the hope that they can send their MPs off in good spirits.

Then on Thursday there are three by-elections simultaneously in which the Conservatives are widely expected to lose all three hitherto comfortable seats. Such is the grim mood for Tories it will be an even bigger sensation if the Tories hang on to any of them.

To absorb the shock waves of these events both the prime minister and the leader of the opposition may well carry out long overdue reshuffles of the cabinet and shadow cabinet teams.

The three by-elections have all been triggered by Conservative MPs who resigned voluntarily, albeit with clouds hanging over their heads. Boris Johnson quit before a by-election could be triggered following his lengthy suspension from the house.

At the 2019 General Election he had a 7,210 majority and 52.6% of the votes.

A close ally, Nigel Adams, stood down in Selby and South Ainsty after failing to receive the peerage which Mr Johnson had promised him. In 2019, he had 60.3% of the votes and a 20,137 majority.

David Warburton had the Tory whip withdrawn last year following cocaine and sex allegations. He finally quit complaining that he had been denied a fair hearing by parliament’s harassment investigators. He had 55.8% of the vote and a 19,219 majority in 2019.

By-elections are often exaggerated expressions of how the voters are feeling about the government.

There have been 13 by-elections this parliament

Early in this parliament, Boris Johnson’s popularity in the so-called red wall constituencies was proved by the Conservative’s unlikely victory in the formerly rock-solid Labour seat of Hartlepool, once held by the New Labour mastermind Peter Mandelson.

The tide has turned since then. There have been 13 by-elections this parliament, many held by the incumbent party, but the Conservatives have lost three seats to the Liberal Democrats and one to Labour.

Labour regained Wakefield after Imran Ahmed Khan, the incumbent Tory MP, was jailed for child sexual assault. The Liberal Democrats won in North Shropshire after Mr Johnson tried to protect his friend Owen Paterson from suspension for commercial lobbying.

They picked up Tiverton and Honiton after Tory MP Neil Parrish quit after being reported for watching online pornography in the Commons chamber. There was no scandal around the death of Dame Cheryl Gillan. She had campaigned vigorously but unsuccessfully on her constituents’ behalf to stop her government’s HS2 railway.

By-elections are useful political weathervanes. More often than not the biggest drops in support for the ruling party at by-elections come in the years leading up to a change of government.

On average, the Conservatives were down 14.1% before Harold Wilson beat Sir Alec Douglas Home, and 19.9% before the New Labour landslide victory in 1997. Harold Wilson was down 17.3% before Ted Heath’s surprise victory in 1970.

General sense is the Conservatives are heading for defeat

Jim Callaghan’s Labour was down 9.3% before the 1979. Gordon Brown was down 10.4% by 2010, although one of the biggest erosions was 19.9% in Labour support in the previous 2001 parliament. It just wasn’t big enough to destroy the massive lead built up under Tony Blair.

The general sense at Westminster is that the Conservatives are heading to defeat at the next election. That explains the pessimism about Tory chances on Thursday.

The well-known American pollster Frank Luntz told a private meeting of Conservative MPs that anyone with a majority of 15,000 or less should be seriously worried about their chances of holding their seat. On that basis, the Conservative candidates in North Yorkshire and Somerset should be hopeful.

Unfortunately for them that is not what polling in either Somerton or Selby points to. By-elections bring additional media attention and voters tend to punish parties if their MP has been forced out in disgrace or bothered them by called an election unnecessarily.

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An upset, which these days means the Conservatives hanging on, looks most likely in Uxbridge and Ruislip, the most marginal constituency of the three being contested. Boris Johnson brought celebrity glamour to suburban London, and some of his former voters still love him and resent him being forced out, as they see it.

The Labour Mayor of London’s plan to extent ULEZ, the ultra-low emission zone, to all of London including the constituency is not popular with many, since it would penalise those with polluting vehicles. The Labour candidate is now opposing his party’s ULEZ plan.

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A surprise Tory victory in Boris Johnson’s old stomping ground would be a mixed blessing for Rishi Sunak. It would be bound to encourage his supporters to claim that Mr Johnson and his polices are what the voters really like.

To prove it Nadine Dorries, another unhappy peerage refusenik, might at last execute her long delayed threat to resign her own seat in Bedfordshire Mid (2019 majority 24,664, 59.8%), plunging the government into further by-election turmoil.

There may well be another tricky by-election for the government soon in Tamworth (2019, Conservatives 66.3%, majority 19,634). A recall petition is due following the suspension of Chris Pincher from parliament for sexual misconduct.

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A similar recall petition is under way in Scotland in the Rutherglen and Hamilton West Constituency following the suspension ex-SNP MP Margaret Ferrier for breaking COVID lockdown rules. If the expected by-election takes place, Labour is hoping to oust the SNP.

Politicians often like to play down opinion polls saying they prefer real votes in real ballot boxes. That is exactly what parliamentary by-elections are. The three results will be scrutinised closely as they come in on Friday, including on Sky News.

They will give a true pointer of which way the wind is blowing and pile conflicting pressures on the party leaders for their reshuffles. To improve their electoral performance should they trim towards the centre or play to their party activists?

Looking at the prospects and the reasons for the elections in Yorkshire, London and Somerset it looks as if tidying up is taking place at the end of a fraying and exhausted government, but it is real voters who will decide next week.