David Davis and Ken Livingstone may not seem to have a lot in common - apart, that is, from their frustrated ambitions to lead their respective parties. Yet they share an unusual distinction among prominent modern politicians. Both of them have resigned to fight byelections on what they saw as issues of principle, only to find that the byelection, although striking and daring in its way, was actually the easy bit.
I write this in advance of last night's result in Haltemprice and Howden, so it is just possible that Davis is waking up this morning to the realisation that he has carried out the most politically suicidal move by a senior British politician since the Labour minister John Stonehouse faked his own drowning off a Miami beach in 1974. In which case, you may have better things to do today than to read much further.
However, on the assumption that the pundits were right and that Davis has indeed been re-elected to parliament overnight, it is important to remember the lesson from Livingstone's now widely forgotten byelection. In 1984, Livingstone resigned his Paddington seat on the Greater London Council in the hope of creating a defining single-issue contest against Margaret Thatcher in which he would be swept back to County Hall on a Save the GLC tide. Yet things did not go as Livingstone hoped. The Conservatives dismissed the contest as a stunt and refused to run a candidate. When the election took place, turnout was embarrassingly low. Livingstone was re-elected with ease, but there was no shot-in-the-arm for the wider campaign to save the GLC, and the council was soon abolished.
I don't think Davis paid enough attention to the lesson from Livingstone's ploy. Yes, Davis caught some people's attention. Yes, it was heartening to see some public re-engagement with the political process. And, yes, it is increasingly likely that the government's 42-day pre-charge detention plan will not now end up on the statute book at all. These are all good things. But this byelection was by most standards a damp squib like Livingstone's.
Perceptions of the Haltemprice and Howden byelection have churned dramatically in the four weeks since Davis stood outside the Commons in the June night and announced he was quitting. Phase one was dominated by the widespread view at Westminster that this was a quixotic act of vanity whose main immediate consequence was to turn the spotlight off Gordon Brown's humiliatingly narrow win in the 42-day vote and on to Davis's enduring rivalry with David Cameron. That was quickly followed by a backlash, disproportionately from the blogosphere, which celebrated both Davis's independence and his cause, and which purported, without much objective evidence and in defiance of most opinion polls, to speak for the mass of ordinary people against the Westminster elite.
That gave way to a dawning awareness that things were a bit more complicated.
The contest in the Hull dormitory zone did not become the political equivalent of a Wimbledon Centre Court final that Davis had hoped for. Labour's refusal to be drawn in - which anyone who knew the Livingstone case might have anticipated - deprived the former shadow home secretary of the setpiece confrontation he needed.
The entry of 25 fringe candidates drastically reduced the election's credibility. Instead of a fight to defend ancient liberties, Haltemprice and Howden turned into a battle to make sure that a decent number of people bothered to vote.
This byelection has been both oversold and undersold. Low turnout, if it has happened, will reinforce that scepticism. Right now, though, the positives also need to be accentuated. From my own brief visit this week I would say these are as follows.
First, the Haltemprice contest undoubtedly encouraged a reasoned debate, in but not confined to the constituency, about the place of civil liberties in the age of al-Qaida.
Second, it gave a small push, which should not be exaggerated, to the public mood on proposals like ID cards, where there has been a clear shift in views over the years, and on 42 days, where Davis is making exaggerated claims about changes in attitudes that are better explained by the kind of questions that the pollsters have asked.
Third, it provided a very practical form of public political engagement (almost certainly more practical than the green paper on this subject that the justice ministry quietly published this week).
Fourth, it made the government's attempt to get the counter-terrorism bill (including the 42-day power) on to the statute book a little bit harder.
It was undeniably impressive to stand in the Hull suburb of Willerby this week and listen to Davis, flanked by the Labour rebel Bob Marshall-Andrews, Liberty's Shami Chakrabarti and the eloquent 7/7 survivor Rachel North, calling on voters to take a stand and make some history. They may not be as big an alliance as they like to make out, but they have real achievements to their name.
But don't let's kid ourselves. It wasn't Davis or a few thousand East Riding voters who delivered the most important blow to the government's plans this week. It was Lady Manningham-Buller, the former head of MI5, in her devastatingly succinct maiden speech in the House of Lords. When the recently retired head of the security services declared that the 42-day power is not justified on grounds of either practicality or principle, the plans were holed below the water line.
Where, by the way, does her speech - and the similar speeches of so many former police, prosecutors and judges - leave those who always claim that "the state" is a sleepless and hegemonic conspiracy against the innocent downtrodden? The truth is far more nuanced than the conspiracy theorists can ever admit.
The 42-day plan is now all but dead. It is clear that the Lords will throw it out when they vote in October.
The larger the majority in the Lords the more difficult it will be for the government - which only won by nine last time - to reinstate the deletion in the Commons in November or to use the Parliament Act to flatten its opponents in the upper house a year from now. With a general election beginning to loom and the economy increasingly at the heart of the political battle, there will soon be neither space nor heart for this fight.
The question is increasingly not when the 42-day plan will fall, but how. David Davis can take some credit for this. But I think it is Lady Manningham-Buller who has fired the fatal shot.
