It used to be an article of faith among Liberal Democrats that general election campaigns would always mean good news for their party. The party’s axiom was that, once balanced election time media reporting rules kicked in, they would get a fairer crack of the whip and increase their support. For many years the facts bore this out. In every election from 1992 to 2015, the Lib Dems saw their share of support increase over the course of the campaign. Leaders including Paddy Ashdown, Charles Kennedy and Nick Clegg became bigger national figures in this way.

That is no longer true. The 2017 election was the first since 1987 in which the Lib Dems did worse on polling day than they were doing when the campaign began. An initial average poll rating of 11% in that election became an actual vote of 7.4% when the votes were cast. The Lib Dem postmortem concluded that the election had come too soon after the post-coalition drubbing of 2015, and that Tim Farron had proved a useless leader.

Events may shift the numbers. But things could get worse for the party, not just better

Yet now the same thing seems to be happening again, although this time the party started from a higher base. In the first eight polls of the 2019 campaign, the Lib Dems averaged a 16 % vote share. In the latest eight, that is down to 13%. True, there is still a week to go. Events, such as Jo Swinson’s interview with Andrew Neil on BBC One on Wednesday night, may shift the numbers. But things could get worse for the Lib Dems, not just better.

Insiders suggest many reasons why things have gone wrong. They range from the overdone original pledge – now effectively abandoned – to scrap Brexit if the Lib Dems, improbably, won a majority, to the overreach of Swinson’s claim that she was running to be prime minister. These were self-inflicted wounds. And Swinson had had a tough campaign even before the grilling from Neil. A poll last month found that voters liked her less the more they saw her.

The reality is that the Lib Dems misread the political mood in the summer. Their successes in May’s English local and European elections, the latter seeing them poll 20% and beat Labour into third place across the UK, marked a change of fortune after the hammer blows of 2015 to 2017. Victory in August’s Brecon and Radnorshire byelection, along with several parliamentary defections, appeared to confirm a shift. As consistent advocates of a second referendum on Europe, the Lib Dems seemed to have regained the right to be heard and to offer an alternative home for Labour and Conservative remainers alike.

In fact, both this right and the offer were more contingent. This was not a return to pre-2010 levels of Lib Dem support – nothing like it. The summer surge was a wave of exasperation over Brexit. But the levels of anger among remainers led the Lib Dems to overplay their hand. They talked of winning 100 or more seats from Tories and Labour alike, when in fact most Tory voters are still leavers and many Labour voters have not forgiven the Lib Dems for the coalition cuts.

When Paddy Ashdown and Charles Kennedy led the Lib Dems into genuine third-party status, between 1992 and 2005, they did so by positioning the party on the centre left as an anti-Tory force. This stance had deep roots, extending back to the Lib-Lab pacts of 1906 and 1977-8 . They also faced a Labour party that, under Neil Kinnock and Tony Blair, shared common political ground with the Lib Dems, including on issues such as Europe, devolution, and even electoral reform. Tactical voting was thus a natural option in those circumstances. The same is not true of today’s Labour party or its leader. Politics has moved on.

Play Video 1:06 Extinction Rebellion bee protesters 'swarm' Swinson battlebus – video

Three conclusions follow. The first is that Brexit has not reshaped the electoral battle as comprehensively as some believe. That is not to belittle the fact that Brexit has done much to recast British electoral politics. This is still a Brexit election, because its results could mark a point of no return on this most all pervasive of current issues. But it is not year zero. The idea that the Lib Dems, by being clear on the biggest issue of the day, will automatically attract all remain voters en masse to their cause is being proved false. It’s as false as Labour’s equivalent fantasy that, by being clear on the need for a radical post-austerity political economy, it will automatically attract all the votes of those who agree with that policy. In both cases, belief in practicality and trust in the leader are crucial to making the sale.

The second is not to make the best into the enemy of the good for the Lib Dems. Even with its campaign setbacks, the party is still polling much better in 2019 than in 2017. It is concentrating its resources in target seats in which there are significant pools of tactical voters. The Lib Dems have come down off their “plague on both your houses” horse: the anti-Tory pitch of “a vote to stop Boris Johnson” is now explicit. Against the background of a decent but not spectacular national share, significant seat gains are highly possible – as some losses may be, including Swinson’s own seat in East Dunbartonshire.

The biggest conclusion is that something will have to change. If the Conservatives win a clear majority next week Labour and the Lib Dems, as well as the other anti-Brexit parties, will still face a strategic question. It is one that has overshadowed British progressive politics many times under the first-past-the-post system, though rarely in as raw a way as it may do next week. “While Liberalism and Labour are snapping and snarling at each other the Conservative dog may run away with the bone,” warned CP Scott, the Manchester Guardian editor, in 1912. After more than a century, it is long past time to stop that happening again.

• Martin Kettle is a Guardian columnist