David Cameron's 2009 description of House of Lords reform as a "third-term issue" does much to explain why so many Conservative MPs will rebel against the government in tomorrow's vote. Reform of the Lords was, Cameron suggested, something a Conservative government would only undertake once it had implemented the rest of its programme. For Tory MPs, his words are a reminder that the bill was only introduced to placate the Lib Dems and that Cameron failed to win an election he should have won. That there is little prospect of the Tories winning a third term (or one term, come to that) is, in their view, even more reason for Cameron to use his time in Number 10 wisely (i.e. to get Britain out of recession, not waste time on liberal fetishes like Lords reform).

The danger facing Cameron as the parliamentary debate begins is not that the bill will be defeated on its second reading (since Labour will support the government) but that the programme motion, which would place a 10-day limit on debate, will be rejected (since Labour, which wants more time to scrutinise the bill, will oppose the government). This would be the first time the government has been defeated on its own business in the Commons and would, in the words of one Lib Dem aide, put the coalition in "uncharted territory". The absence of a time limit for debate would allow MPs to filibuster the bill and would delay the rest of the government's legislative programme.

If the bill does become marooned in the Commons, one possibility is that the government will agree to a referendum on the subject. Labour has already called for one and at least some of the Tory rebels (such as Nadhim Zahawi and Rory Stewart) also support a public vote. For the latter, flushed with success from the AV campaign, a Lords referendum is another chance to give Nick Clegg a bloody nose.

Clegg has always insisted that a referendum is unnecessary since all three of the main parties supported Lords reform in their manifestos. But he would find it hard to argue that the people should not decide if parliament is divided. One suspects that Cameron, who has left the door open to a referendum, will look again at this option if the rebels carry the day.