It’s an unwritten rule of politics that every electoral action can have an opposite – if not quite equal – reaction.

The question being asked by many Tories this morning will be whether the party’s recent relentless focus on winning new supporters in traditionally Labour-held areas in the north is starting to cost them in their strongholds in the south.

As ever, the reality is more complex.

Chesham and Amersham presented a potent mix of local and national issues that the Lib Dems were able to capitalise on.

The HS2 rail line runs through the constituency and proposed planning changes are a big concern here too.

And as the losing Tory candidate has said, the Lib Dems threw the kitchen sink, the microwave and everything else at this seat.

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Tactical voting may have also been at play with Labour voters switching to the Lib Dems to ensure a government defeat.

But all that said, this trend – of traditional Tory regions in the south being eaten away by the opposition parties – was visible in May’s local election results.

David Cameron’s old Oxfordshire constituency of Whitney and the bastion of moneyed Home Counties Toryism Chipping Norton both got Labour councillors in that vote.

Meanwhile further south in Sussex, the conservative leader of Worthing Borough Council explicitly blamed the focus on levelling up in the north and prevalence of planned housing developments in the south for a loss of seats in his region.

The Lib Dems are echoing that this morning, saying that voters in places like Chesham and Amersham are fed up of being taken for granted.

Demographic change as young families move from inner London, along with chunky support for remaining in the EU in 2016 may also have played a part.

The messaging from Tory sources this morning is that this is a predictable mid-term protest vote against a party that has been in power for ten years and the seat can be retaken at a general election.

That may be right – but remember, that logic wasn’t borne out in the Hartlepool by-election where the Tories took the seat in May.

It also comes at a time when the government is polling well, in the midst of a successful vaccine rollout.

All of that means this is a more surprising result than Hartlepool.

But just because this brick has turned yellow, doesn’t mean the whole blue wall is guaranteed to come falling down.

For a start, there are genuine questions about whether there is a big enough crop of seats the Tories would realistically lose in a general election to counter their wins in the north.

What’s more, while in the north the Tories are now single-handedly taking votes from Labour, in the south the spoils of disillusioned conservative voters are more frequently being shared between a number of opposition parties.

For Labour, this is bad news.

This by-election saw the opposition vote collapse to just over 600 votes.

May’s bumper round of polling also saw good gains for the Green Party, as well as the Lib Dems.

Expect questions for Sir Keir Starmer today about why Labour is failing to win in places like Chesham and Amersham when the Lib Dems are.

The practical question for the next general election may be whether anti-Tory electoral pacts are the only way to get close to pushing Boris Johnson out of power.

Countless prime ministers have learnt the lesson of taking voters for granted the hard way.

It’s an irony not lost on Tories that the same sense of disillusionment they are trading off in the north now appears to be costing them votes in the south.

This is a stunning result for the Lib Dems and while it’s too soon to talk about the southern blue wall crumbling, it’s certainly wobbling.