Political theatre, by its very nature, distracts from the substance of politics. So it is worth emphasising that nothing David Davis said or did yesterday changed the basic argument over detaining terror suspects without charge. The Bill that would allow detention for up to 42 days is wrong in principle, unworkable in practice and has been forced through the House of Commons by means of brazen political chicanery.

Mr Davis agrees with this. As Shadow Home Secretary he was ideally placed to say so. He could have continued to expose the Bill's absurdities as it worked its way through Parliament, and fought to prevent the Government enforcing it as law in defiance of the House of Lords and - as he has noted - Magna Carta.

Instead he has resigned to force a by-election that he hopes to fight on this one issue. Labour, baffled but delighted, may let him do just that. If so, it can be expected to present the 42-day measure, however misleadingly, as a proxy for toughness on national security. But Labour could equally attack Mr Davis's character. By espousing precisely the gesture politics for which he rightly condemned the Government 24 hours earlier, he has forced his own flawed judgment, erratic temperament and unrestrained ego on to his constituents' agenda.

However the Haltemprice & Howden by-lection plays out next month, it will be the result of one of the most egregiously self-serving political stunts in living memory. Seldom has a politician turned himself so pompously and effectively from a national figure to a party embarassment with a single passage of weary rhetoric. If the Prime Minister managed to transform himself in a few weeks last year from Stalin to Mr Bean, Mr Davis has transformed himself in a few minutes from bruiser to loser.

His position on 42 days is not contrived; he is a sincere defender of civil liberties. But his resignation is grandstanding of a high order, and self-defeating at that. Tactically, it has thrown the Conservatives from the front foot to the back, as evidenced by the justified crowing of Labour ministers yesterday at his “childish” behaviour. Strategically, Mr Davis may hope to give the Lords a popular mandate for blocking the 42-day plan with his probable re-election, but what attention it attracts is more likely to focus on his own Quixotic figure and the cost of the whole peculiar event to the taxpayer. More broadly, his resignation is a betrayal of his party leader's trust and a reckless rejection of the logic of party loyalty.

The Liberal Democrats' decision not to field a candidate against Mr Davis makes his return to the House of Commons almost inevitable. It also creates the outlines of an unofficial Tory-Liberal Democrat alliance on civil liberties. But David Cameron suddenly has more pressing matters on his mind. His task since the Conservatives' thumping local election victories last month has been to erase lingering perceptions of his party as the permanent Opposition and recast it as a government-in-waiting. He had been doing well. Until Wednesday night, the party had not looked so disciplined since Thatcherism's heyday. But he was powerless to stop Mr Davis, who will soon cast a brooding and divisive shadow from the backbenches.

By resigning, Mr Davis has shown contempt for the principle of Cabinet solidarity, without which parties cannot effectively govern or hold each other to account. He has shown indifference to Parliament's primacy in passing laws (however flawed). And he has raised serious questions about the judgment of the rest of the Tory front bench for tolerating him for so long.

Mr Cameron and the nation have this consolation: Mr Davis has won the headlines that he clearly craves, but he has disqualified himself as a future home secretary.