By Nick Assinder
Political Correspondent, BBC News website
Government proposals to introduce ID cards, now ping-ponging between the Commons and Lords, have run into plenty of opposition since they were first revealed.
A number of Labour backbenchers, Liberal Democrats and the Tories have united to attack the plans on a number of different fronts including cost, civil liberties and likely effectiveness.
Ministers accused of back door compulsion
Meanwhile, ministers have claimed they are needed to help combat everything from terrorism to social security fiddling to identity theft.
But the scheme has now run into what is likely to be its final battle - between the two houses of parliament - and it has all boiled down to the issues of compulsion and Labour's election manifesto.
The clash came after the Lords continued to throw out the part of the proposals that would force anyone applying for, or renewing a passport to get an ID card and sign up to the planned national identity database.
Ministers have managed to overturn the Lords' amendments each time the bill has come back before MPs and the proposal is now stuck in the game of ping pong with ministers threatening to invoke the Parliament Act in November to force it onto the statute books unless peers relent beforehand.
Opponents argue that by forcing passport applicants to get a card, the government is introducing compulsory ID cards by stealth and that is breaking the Labour manifesto commitment to make the scheme voluntary.
Home Secretary Charles Clarke has rejected the argument, insisting during a heated Commons debate, that nobody is forced to have a passport, so the scheme remains voluntary.
Clarke was jeered by MPs
It was a claim, however, that was greeted with laughter and jeers from opponents, with Tory Edward Garnier claiming it amounted to "intellectual dishonesty on a grand scale".
Liberal Democrat spokesman Nick Clegg said the flawed ID card plan was bad enough, "but imposing it on the British people makes it worse".
And even some supporters of the proposal are opposing it thanks to the claimed element of compulsion.
But Mr Clarke has again insisted the government has no intention of backing down on the proposal, effectively challenging the Lords to end their opposition.
If they continue to refuse, peers will risk sparking a constitutional battle which might well end with the government forcing it into law via the Parliament Act.
But even that could lead to another battle over what was actually in the manifesto - and what it meant.
What appears clear, however, is that the prime minister remains as committed to this proposal as ever and critics' chances of killing it off for good still appear slim.