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Yesterday's Head to Head on the Sunday Politics on House of Lords reform between Tory trouble-making MP Philip Davies and a maverick Liberal Democrat peer Matthew Oakeshott might seem like an amusing sideshow to mainstream politics.

But later in our Week Ahead segment described it as the shape of coalition politics to come.

I suspect he could be right.

The crucial part of the Davies-Oakeshott spat came when Davies said he and liked-minded Tories would block Lords Reform in the Commons and Oakeshott shot back: "then you'll fight the next election on the existing boundaries".

There's the rub. Nick Clegg, who has made Lords reform his personal project, knows he has an uphill struggle.

There are plenty of Tory peers opposed to reform and Labour peers are unlikely to be helpful.

Indeed there's even a bunch of 20 or so Lib Dem peers who don't much like reform either.

It's accepted that when it comes to constitutional issues like Lords Reform Messrs Cameron, Clegg and Miliband don't have full control over the peers who take their parties' whips.

But what if Lords reform was to be derailed by a Tory rebellion in the Commons (while Labour MPs sat on their hands)?

Well, that would be another matter for the Lib Dems. They think they have a deal with the Tories: help us with Lords reform and we'll back the boundary changes which will cut the number of Commons seats by 50 (with Labour and Lib Dems losers and the Tories gainers).

Hence yesterday's Oakeshott threat to the Tories: renege on Lords reform and you can kiss goodbye to the boundary changes.

Put aside the idea that many Tory MPs thought they'd agreed to the AV referendum in return for the boundary changes.

And the other thought that, for personal reasons, a number of Tory MPs aren't that keen on the boundary changes anyway. Both may be true or untrue.

What matters here is that the Cameron high command is pretty sure it has no chance of winning an overall majority at the next election without the boundary changes in place. We're talking big stakes here!

If the PM cannot deliver his own troops for Lords reform and the Lib Dems retaliate by scuppering the boundary changes then that could fracture the coalition beyond repair.

That's by no means certain but it is the fissure around which the coalition will increasingly be divided, as Janan said on Sunday.

Remember it is often the unexpected that rocks politics, including recondite constitutional matters.

The Callaghan Labour government didn't lose a motion of no confidence in 1979 (thereby causing the election it would go on to lose) over the Winter of Discontent, on which the whole country was focused; it was a spat over devolution.

And General DeGaulle was not ousted by the student-worker uprising of 1968.

He had to step down after botched constitutional reforms in 1969. So Lords reform is not just a sideline concern for Westminster wonks.

It could be the issue that rocks the coalition.