There's an article at Open Democracy from the Scottish Green activist Gabriel Neil, outlining his reasons for agreeing with the conventional wisdom that there will be no SDP-style split in the event of Jeremy Corbyn becoming Labour leader. I must say I don't find them hugely convincing. Basically they boil down to the idea that, unlike in 1981, the conditions do not exist for a new centrist party to become electorally successful.
"The circumstances which allowed the SDP to become a party which rocked the political boat in the early 80s (some polling even predicted they would win the 1983 election) do not exist for the Labour right any more...They do not have two extremes to oppose themselves to – they are part of a cosy consensus with Tory ideology which has led millions to stop voting altogether...The Labour right simply do not look like an exciting electoral prospect on their own, and I suspect they themselves know it."
The fact that much of the Labour parliamentary party is ideologically closer to David Cameron than to Jeremy Corbyn is precisely the reason why the situation that is seemingly about to arise will not be sustainable. These people are not going to be able to stomach sitting on the 'wrong' benches indefinitely while, for example, Cameron defends NATO membership and Corbyn agitates for withdrawal. I'm sure the Labour right are for the most part sincere in saying that they don't plan to leave the party, but that's because they're frantically telling themselves that the Corbyn era will be a very brief blip. They may even be right about that. But if the hard left bed themselves in, and if there is no realistic prospect of replacing Corbyn with a leader who is not a Corbyn protégé, the unthinkable will swiftly become thinkable, and the right will start to look for options outside the official Labour fold.
Just before she helped set up the SDP, Shirley Williams floated the idea of a 'unilateral declaration of independence' by the Labour parliamentary party. That sort of option may be revisited - it would look like an elitist coup, but the Labour right may see it as the least worst option, because what would effectively be a new party (and the Electoral Commission would probably force it to adopt a new name) might be perceived by the public as the de facto successor to Labour. The chain of events could be something like this -
1) Mutterings after electoral setbacks.
2) Careful establishment of two narratives : First, that in a parliamentary system, no party leader can remain in office without the confidence of his or her parliamentary party. Second, that it is in the overwhelming national interest of Britain to have a credible opposition to the Tories - ie. the primary loyalty of Labour MPs is to the British people, not to the "unrepresentative minority" who elected Corbyn.
3) A PLP-only vote of no confidence in the leader.
4) The PLP votes to unilaterally declare its independence after the leader ignores no confidence vote. A new party is effectively created, leaving behind a rump 'official Labour'.