If the motion is won, Cameron's traditionalists will become more vigorous in their argument that the Liberal Democrat tail is again wagging the Tory dog. If the vote is lost, Nick Clegg's left wing will look to exact revenge for what they will see as the Tories reneging on the coalition agreement.
More of the same? Not quite. As with written constitutions, so with the agreement: the same words can mean different things to different people. But there's no arbiter to decide what the agreement means. The perception of a commitment broken is far more corrosive than support withheld.
There's another difference. I've long thought that the coalition would last for the full five years on the basis of pragmatism: neither leader would relish going to the polls part way through this Parliament. The trick would be whether they could carry their parties with them.
Not only is that getting more difficult, but Lords reform is something to which Clegg visibly attaches more importance than other issues. So failure here for the Lib Dems would be doubly harsh.
But maybe debate about coalition tensions focuses on the wrong point. We need a second chamber that will hold the executive to account. Select committees have failed to do so, preferring easy headlines by attacking non-government witnesses. The Government's proposals for civil service reform will concentrate executive power. Where are the checks and balances?
On second reading of the Lords Reform Bill, Jesse Norman pointed out that 'the Blair government was defeated four times in the House of Commons and 460 times in the House of Lords'. Maybe that sets a future benchmark?