Andrew Rawnsley writes in this morning’s Observer that Labour and the Conservatives are in truth at least two parties each, held together only by the exigencies of our electoral system.

The rise of Jeremy Corbyn and the increasingly vicious language are explained by the fact that “The big truth that is being exposed by this battle is that Labour is really two parties and they can no longer stand each other’s company.”

That much seems perfectly clear, yet despite that the embattled Blairites are taking great pains (£) to emphasise that they’re not considering jumping ship. Is this wise?

One of the strengths of our electoral system, to those who support it, is that it forces would-be political coalitions to form and negotiate their programme before the election. Supporters of PR who bewail the absence of coalition politics often miss the point, which Rawnsley grasps, that our big tent parties are alliances already.

An advantage of this state of affairs is that each party marries pragmatists with ideologues, helping to prevent a hegemony of the mushy, technocratic centre. But a downside is that it can make such coalitions inflexible, locking individuals inside parties that don’t best suit them.

This problem is especially acute for Labour, whose internal culture demands a devotion to the party that would be completely alien to the more pragmatical Tories.

Yet there must surely be a limit to the proper extent of loyalty to any party.

Whatever you think of them, the Chancellor’s bold moves onto Labour territory in the latest Budget mean that the gap between the Labour right and the Conservative left, or even the leadership, is far from unbridgeable.

Many Blairites, such as the Independent’s John Rentoul, already admit to having more in common with Cameron than the current debate inside Labour, whilst Frank Field has this morning compared George Osborne to none other than Clement Attlee.

If Corbyn were to win, and set about drawing up an Opposition programme which they believed to be contrary to the nation’s interests, at what point would Labour’s right draw the line?

Would they seek re-election under the auspices of what ex-Militant MP Dave Nellist has dubbed ‘New Old Labour’, on a far left manifesto?

If the MP for Islington North does put Labour out of power for another two or even three elections, should talented people who fundamentally oppose his programme squander themselves in opposition rather than serve alongside other moderates in blue rosettes?

The easing of the postwar two-party hegemony harkens back to a much more flexible era of British politics in which ‘crossing the floor’ was a far more common occurrence. With the SDP standing as an object lesson in the risks of setting up a whole new party, perhaps the end result of this dire talk of ‘splitters’ might be a revival of that practice.