After the seismic shocks of 2016, the year draws to a close with the folk of Richmond Park serving up something that feels more normal. They have given us a Lib Dem byelection upset. The airwaves thrum to talk of “a shockwave” for the prime minster from triumphant Lib Dems. The response from the government is the customary shrug that one byelection “changes nothing”. It is like the old days again. Before 2010 and the Lib Dem decision to go into coalition with the Conservatives, it was a deeply rooted tradition of British politics that disgruntled voters expressed their discontent with the larger parties by surging to the amber crowd at byelections. The Lib Dem byelection shock was one of the release valves of our unwritten constitution. It was as stitched into the fabric of British political life as the state opening of parliament.

Sometimes, the consequences have been worthy of the description “sensation”. When the Tories lost Eastbourne to the Lib Dems in October 1990, it finally forced a conclusion on the minds of Tory MPs. The next general election was lost for so long as Margaret Thatcher remained leader. Eastbourne was a significant contributory factor to her defenestration from Number 10 a few weeks later.

The result from this leafy patch of south-west London is not a category one byelection. It is not going to change the prime minister. Richmond Park belongs to a lesser, but still significant, tier of byelections. These are results that change the political atmospherics and may be a harbinger of deeper realignments to come.

Tim Farron joyously proclaims: “We’re back.” It certainly looks true that the Lib Dems have returned as a force to be reckoned with by the other parties in some areas of the country. Under the radar of most of the media, Mr Farron’s party has been displaying twitches of life since the summer. They have clocked up gains in local council contests and secured a 19% swing in their favour at the expense of the Tories at the Witney byelection. The even better swing achieved by Sarah Olney in Richmond Park is the best evidence yet that the Lib Dems can attract support from voters in prosperous areas that want to stay in the European Union. Their decision to frame the Richmond Park contest as a mini-referendum on Brexit was clearly an astute call in an affluent seat that voted 69-31 for Remain. It was an additional bonus that the Tory incumbent was an Outer and Zac Goldsmith’s disguise as an “independent” was so see-through.

This will lift Lib Dem morale and likely boost the attention they receive from the media. That will influence the shape of the national conversation, especially about Brexit. The producers of the Today programme may be less inclined to groan when his publicists offer Mr Farron for interview, and bookers for Question Time may now be more minded to include a Lib Dem on the panel.

It will encourage the Lib Dems to double-down on their strategy of being the unequivocal voice of British pro-Europeans. According to the pollsters, only about a quarter of the total electorate say they agree with the Lib Dem call for a second referendum on Brexit. So that is a niche appeal. But for the Lib Dems, that is a very substantial niche, much larger than their current national poll rating.

I am warier of the claim that Richmond Park advertises the advantages of trying to broker the agreement of a broader “progressive alliance” to take on the Tories and Ukip. The Greens were smart to stand aside from this byelection. They weren’t going to win and their vote would have been squeezed. Their reward for getting behind Ms Olney is to bask in some reflected glory. There were Labour figures who urged their party to do the same and stay out of a contest they could not win. This was rebuffed by the party’s leadership. When this byelection was first called, I suggested that this probably wouldn’t make much difference because British voters have loads of experience of byelections and understand the dynamics. So it proved. The Labour vote was crunched down to a deposit-losing level as Labour supporters switched behind the Lib Dem. Labour attracted fewer votes than it has members in the seat.

This indicates that Labour supporters are once again prepared, as they were not during the coalition years, to lend their votes to the Lib Dems when that is the best option for defeating a Tory. Tactical voting is back. It may also suggest that a “progressive alliance” is not going to come about as a result of a grand bargain imposed from above by party leaders, especially not when there is so little enthusiasm for that idea at the top of Labour. If a “progressive alliance” does take form, it is more likely to be the result of something more organic growing from the bottom up at constituency level.

Richmond Park will not stop Brexit, but it does throw further grit into Mrs May’s already hellishly fraught calculations. After months when debate within her party has been dominated by the hard Brexiters, Tory pro-Europeans are perking up. They can cite the byelection as evidence that this is not an electoral one-way street for the Conservatives. Richmond Park suggests that many Remain voters are not reconciled to Brexit, they can channel their anger electorally at the expense of the Tories and will be even more likely to do so if Mrs May pursues the harder versions of divorce from the EU.

Her working majority in the Commons has just been reduced to 13, a reminder of the fragility of her position in parliament even before she has unveiled her plan for Brexit. Given the choice, no prime minister in full possession of her wits would want to embark on the most complex and contentious negotiation in our postwar history with such a precarious position in parliament. That argues for Mrs May cashing in her robust lead in the national polls and seeking a mandate while the economy is still relatively stable.

Against that is her aversion to risk and the substantial number of Conservative MPs who will fear a Lib Dem revival, especially in seats where there is a substantial Labour vote for Lib Dem challengers to squeeze. Quite a lot of the seats that the Lib Dems used to hold before the 2015 wipe-out now look winnable again. One clear consequence of Richmond Park is that Conservative MPs in Tory/Lib Dem marginals will become much more interested in the national poll rating of the Lib Dems. They will be watching that closely to see if the byelection win translates into a broader improvement in the party’s position.

It will give heart to Remainers, but they shouldn’t lose their heads

Cheers for Ms Olney’s victory have not been confined to the ranks of her party. For non-Brexit Britain more generally, it has been a tonic in an otherwise bleak year. But pro-Europeans need to be careful not to overstretch their interpretations of this result. A Remain constituency voted for a Remain candidate. That will give heart to Remainers, but they shouldn’t lose their heads. A Leave constituency voting for a Remain candidate would be much more interesting because it would be evidence of remorse among Brexit voters. That day could come if the economic consequences of Brexit are nasty and/or the terms of the divorce are palpably to the disadvantage of Britain. That day is not here yet.

Many parts of Britain are not at all like Richmond Park. This Thursday, we will get the result of another byelection, the contest in Sleaford and North Hykeham, a very different part of England. I am not taking much risk with my reputation as a forecaster when I confidently predict that the Lib Dems will not win this Lincolnshire seat. Its vote in the referendum was a near mirror image of Richmond Park. Sleaford and North Hykeham cast 62-38 for Leave. Much of the interest here is whether Ukip can start making good on the claims of its new leader, Paul Nuttall, that the party can hoover up working-class Labour voters. Labour was second in the seat at the general election, with Ukip less than 1,000 votes behind. If Ukip overtakes Labour, it will be panic time among the party’s MPs. Many are already fearful that Labour will be scrunched from both ends as the Lib Dems steal middle-class, pro-Remain voters in the south and the cities while Ukip eats their working-class, pro-Brexit voters in the north, the Midlands and Wales.

A strong performance by Ukip in the Lincolnshire contest will have more people asking whether we are in the process of a big realignment of British politics. Is the old divide between left and right being transcended by a new faultline between Remainers and Outers?

The honest answer is that we don’t yet know. All we are doing at the moment is gathering clues. What we can say is who stands to lose and who to gain if this is happening. That polarisation would help the Lib Dems, the unequivocal party of Remain, and profit Ukip, the united party of Leave. For the larger parties, both split over Brexit, it would have nightmarish implications. For the Tories, it would be bad. For Labour, it would be worse. I’m not going to guess what result we will get from Sleaford and North Hykeham. I do suggest it will be worth staying up for.