Evidence for this tendency towards splinter groups on the political left is scattered throughout history. One only has to think of the SDP split from Labour in 1981. However there are more recent examples.
If you wish to vote for a progressive candidate in England in the upcoming general election in most constituencies you will have the choice of Labour, Liberal Democrat or Greens. In Scotland, ignoring the constitutional question for the moment, you can add the SNP into the mix; and in Wales, Plaid Cymru will also make it a choice between four.
Compare this to a Conservative party on right whose polls suggest that most previous UKIP voters have consolidated around them and you’ll see a united right that will beat a divided left any day.
That’s not to say that there aren’t significant and important difference between these parties, but these difference are not something that the first past the post system accommodates.
The unity verses disunity problem that the left has extends to the internal politics of the two largest parties. Whether you blame Jeremy Corbyn for persisting despite a lack of support from his MPs, or you blame those MPs for not rallying round the leader with majority support amongst the members, one thing is clear: the Labour party itself has a unity problem.
This is in sharp contrast to the Conservatives. Despite Theresa May having only recently fought a leadership contest where she by no means had universal support you would now be hard pushed to find a Tory MP willing to speak about Theresa May in anything other than the most unequivocally positive terms.
This one basic problem that people on the left can’t seem to consolidate around one person or party is causing progressives major issues. So if Labour has unity problem and voters on the left more generally have a unity problem, it is therefore baffling that Labour supporters in Northern Ireland would like to see Labour candidates adding to an already crowded left field over here.
I understand that the Northern Irish voting system for Stormont elections encourages a wider range of parties but when it comes to Westminster we must suffer under the same first past the post as everyone else, meaning a split vote is a weak vote. The parties on the right here understand this fact and have countered it by being pragmatic enough to form pacts, doing the one thing parties on the left aren’t prepared to do: compromising in order to gain power.
Compromising your views is never an easy thing to do but I’m sure in the UK most Green voters would much rather see Labour in power than the Conservatives, and in Northern Ireland most Labour supporters would much rather see the SDLP winning seats than the DUP.