If, as seems likely, as many as 100 Conservative MPs rebel over House of Lords reform , the bill's fate will depend on Labour. For Ed Miliband, this creates a political dilemma. Should he exploit an opportunity to maximise coalition tensions or should he fulfil Labour's previous commitment to an elected upper chamber? The answer, it appears, is that he will do both.
Today's Guardian reports that  Labour will reject any timetable for the bill, potentially allowing MPs to talk it into the ground. However, it will do so on the basis that the proposed bill would not create a 100 per cent elected chamber (20 per cent of members, including Church of England bishops, would be appointed) and that it would not be put to a referendum. This, Labour will say, is another "miserable little compromise " from Nick Clegg.
Both David Cameron and Clegg have insisted that a referendum is not required since all three of the main parties endorsed Lords reform in their manifestos. But Labour can point to the fact that its manifesto also included a commitment to a referendum. Until recently, the British electorate had little experience of referenda. The AV referendum was only the second to be held on a national level (the first was the vote on EU membership  in 1975). But that vote - and those on directly-elected mayors - have set a precedent. Once the possibility of a referendum is raised, it is hard to argue that the people should be denied a say.
The Lib Dems, however, will say that this is merely another example of Labour's constitutional conservatism (as previously demonstrated during the AV referendum). If the choice is between an 80 per cent elected house and a fully appointed one, then it would be shameful for Labour to side with the status quo. The priority is to establish the principle of election. A fully-elected chamber is a fight for another day.
Were Labour to sabotage reform all the same, then, as Rafael wrote recently , "an important symbolic threshold" will have been crossed. For the first time, on a matter of substance, Miliband's party will have sided with the Conservatives against the Lib Dems. But for Labour, which is increasingly confident of winning a majority at the next election, there may now be little incentive to woo Clegg's party.