One of the things that the Liberal Democrats will miss about Vince Cable when he's gone is his willingness to say colourful things in order to get some coverage – any coverage – of his party. Last year he raised eyebrows by describing Theresa May as “being handcuffed to a radiator in the basement of a flat in Beirut”. This year's headline-grabbing and unwanted mental image is that of the “erotic spasm” that leaving the European Union generates among the hardcore Brexiteers in the Tory party who have Theresa May hostage (whether this in Beirut or elsewhere is unclear from the pre-released extracts).

The Twickenham MP will also call on May to surprise everybody by coming clean about Brexit and backing a referendum on the terms of the deal. Cable believes that casting May as an object of pity – he will tell conference delegates he is “starting to feel sorry” for the Prime Minister due to her Brexit predicament – is the most effective way to damage her politically. There will be some choice words about Labour too, but the Liberal Democrats still haven't quite found an anti-Corbyn theme they are as fond of as the “pitiful May” meme.

But even if they do, the state of the two big parties isn't quite the opportunity it might look at first blush. The big Liberal Democrat message is that the Conservative Party is in the grip of its fanatical wing and that Labour has been taken over by extremists. Almost everybody agrees with at least one of those, but the problem is that hardly anybody agrees equally with both statements. Most would either rather risk a Corbyn-led Labour government than a Conservative one under Boris Johnson, or vice versa.

Adding to that problem is the Liberal Democrats' self-denying ordinance about going into coalition again. The party's message, as far as I can tell, is that the two main parties are backwards-looking extremists who can't be trusted but the Liberal Democrats won't stop them; that in exchange for some rail improvements in Cornwall and a second referendum they will let Jacob Rees-Mogg or John McDonnell do whatever they want with the powers of the executive. (It's particularly eccentric in the case of Labour, as most of the common objections that Liberal Democrats raise with Corbyn tend to be around what he would with the power of the government, not the laws he would seek to pass in parliament.)

I know I'm at risk of repeating myself but it highlights the problem I pointed out yesterday: that by declaring they won't take office after the next election it invites the inevitable question of “so what are you going to do about it?” whenever they slag the two big parties off. Plenty of people are willing to believe that there are lunatics at the wheel of British politics. Far fewer are going to reward a party whose plan to deal with that is to lock themselves in the car boot.