Early May’s huge electoral tests are now nearly upon us. They remain, however, shrouded by claim and counter-claim within Labour ranks as to what the results will actually mean. Labour’s old Centre and Right, still nothing like reconciled to Jeremy Corbyn’s leadership of the Party, contend that Labour must make several hundred gains in English local government, in line with historic averages, even to hope of making any progress in the next General Election. Mr Corbyn’s defenders say that any rise in the Party’s vote from its disastrous performance in May 2015 (when the party gained a 31.2% share of the vote outside Northern Ireland) would point to progress.

There seems to be slightly more consensus surrounding the outcome elsewhere in the United Kingdom. Both sides in Labour’s internal debates are braced for widespread losses in Scotland, but Labour’s parlous situation there is clearly the product of many years of deep-seated Labour decay and Nationalist advance. The position in Wales, where Labour and the Conservatives are both under threat from an insurgent United Kingdom Independence Party, is rather less clear, but alienation from all forms of ‘established’ politics, and the effect of the approaching referendum on Britain’s membership of the European Union, are clearly again here factors beyond Labour’s immediate control.

So what is likely to happen, and what is a good test of Labour’s electoral performance at the moment? We can start with opinion polling and local by-election performances since Mr Corbyn was elected Labour leader, and then build up a likely picture from there. Neither have been very good at all: indeed, Labour polling was absolutely dreadful until the Conservatives’ self-inflicted implosion of the last month or so, starting with the issue of disability welfare payments in the Budget, and then taking in divisions over Europe, the resignation of Iain Duncan Smith, the potential closure of the Tata steel works in South Wales and then the embarrassing revelations contained in the Panama Papers.

Now, Labour’s numbers have got a little better: they are ‘only’ 3.8% behind the Conservatives if we take each pollster’s last numbers into account. In February, they were an average of 8.4% behind, while at this point in the last Parliament Ed Miliband’s Labour Party actually led the Conservatives by something like 5%. But in local by-elections held every Thursday, they have as yet shown very little progress: in this calendar year, and in contests where we can compare Labour and Conservative vote shares with the last time the war was fought, the Labour vote is down about 2% on the last Parliament, with a 1% or so swing to the Conservatives.

None of this is particularly encouraging, and the acknowledged experts in this field, Michael Thrasher and Colin Rallings from Plymouth University, think that Labour might lose about 150 English council seats in May. Labour’s very recent and somewhat marked progress in the polls might allow them to do rather better than this, and the local pattern will actually be as interesting as the headline results. In 2012, UKIP hugely underperformed on its share of the popular vote where they fielded candidates (about 13%): but they gained hardly any councillors, and made no net gain in seats. That is likely to be very different this time, and UKIP might hurt Labour – either by gaining seats themselves, or allowing the Conservatives to do so – in areas where their intervention seems to have greatly reduced the Labour vote in 2015: Harlow, Plymouth, Southampton and Thurrock.

Elsewhere, Labour will face tough challenges from the Conservatives in tightly-contested councils such as Crawley and Redditch, while if by-elections and detailed polling are any guide, Labour might have a rather better night in smaller, radical and heavily student-focused cities such as Cambridge, Norwich, Oxford and Exeter. A poor night even here would perhaps be the greatest cause for concern.

Bear in mind here that performance in such elections is an enormously powerful indicator – perhaps one of the most powerful signals – of the next General Election’s actual result. Matt Singh of Number Cruncher Politics reckons that a Labour defeat on the scale Rallings and Thrasher project would mean, on historic trends, a 2020 Labour loss to the Conservatives by something like 10%. On a uniform swing, that would mean a Conservative majority in a ‘reformed’ 600-seat House of Commons of something like 70 or 80, Labour having been reduced to no more than 200 seats in that slightly smaller House. Only twice in modern times – 1982 and 1985, both years in which Labour could be fairly described as in disarray – have Oppositions lost seats in local elections. No Opposition in that time has ever made a net loss of seats and then gone on to win the subsequent General Election. If Labour really does make more than a hundred losses – by no means a certainty – then the Party should be very worried indeed.

The picture is if anything worse in Scotland, and the debacle of 2014-15 seems not to have receded. Labour’s move to the Left there, advocating tax rises to mitigate the impact of ‘Tory austerity’, has done little to counter the Scottish National Party’s dominance of Scottish civil society as well as the mantle of calm, competent, centrist good governance. The last three polls in Scotland give us an average of SNP 53%, Labour 20% and the Conservatives 16% on the constituency-only vote: enough, when combined with the regional list numbers, to give Labour only 26 seats, down eleven even on their 2011 drubbing – and involving the loss of all of their constituency MSPs. Though Labour will probably just beat the Conservatives to second place and the status of official Opposition, that result is not assured: results from the latest YouGov poll would probably give the Scottish Conservatives 24 MSPs, and Labour only 23. Ruth Davidson, the Conservatives’ leader at Holyrood, is popular, confident and making an explicit pitch to be Leader of the Opposition: if she gets her way, it will be yet another devastating blow to Scottish Labour.

In Wales, the latest YouGov Political Barometer poll holds out at least some respite to Labour: they seem unlikely to suffer enormous losses, and indeed over the last couple of months the Conservatives seem to have fallen back quite markedly in this contest. Roger Scully of Cardiff University thinks that Labour might gain 28 or 29 Assembly Members on these figures, down one or two from 2011. That would be disappointing, though it would not represent as bad a result as 2007, when the Party’s numbers in Cardiff Bay went down to 26 AMs. One caution here, however: in the equivalent poll back in 2011, Labour’s vote was running at 49% for constituency seats, and 44% on the regional list: they actually got 42% and 37% on the day. Now those polling figures are 35% for constituencies and 31% on the list: under-performing their polls to the same extent would see Labour suffering easily its worst Welsh result ever. Carwyn Jones, Labour’s leader in Wales (and YouGov) will hope that nothing like that gap replicates itself this time: recent methodological changes made by the pollster should close some, if not all, of this discrepancy.

Only in London does Labour seem assured of success. Here a strong and flexible campaign from Sadiq Khan, distancing himself ever-so-slightly from Mr Corbyn while keeping the latter’s supporters on board, and a lacklustre effort from Zac Goldsmith as the Conservative campaign, seems to have combined with favourable demographics and a membership boom to buoy Labour to victory. The latest polls give Mr Khan a lead of between 8% and 10%: losing this contest would now be a major shock, suggesting that Labour cannot compete even in Mr Corbyn’s metropolitan heartland. Winning London back from a certain Boris Johnson will give Labour something real to cheer, whatever happens elsewhere. As Londoners seem to have warmed to Mr Khan over the last few months, so Labour’s leader has seemed rather more secure in his job.

When you strip away all the complexity, Labour actually has a very simple task: to move forward on 2011, and the equivalent moment in the last Parliament, and on 2012, when the English local elections were last fought in these same areas. Only then can the Party show that it is anything like on course to even be part of the next government.

Standing still is not much of an option. If they can somehow scrabble to the same number of AMs, MSPs and councillors as last time, they will be demonstrating that they may be able to hold the line and avoid a really bad defeat in 2020, but no more. If they do even worse and go backwards overall, and their representation falls back everywhere outside London, this will only serve to confirm what polling evidence and local by-elections have been telling us all along: that the Labour Party is on course for a very bad defeat in the next General Election.

That outcome is hardly inevitable, as the last few weeks have demonstrated. It is not even really clear that the data we have so far makes it very likely, rather than just probable. That’s why May represents such a good test, and its results such a good signpost. Some retreat in Scotland and Wales seems extremely likely. That makes England, where most of Labour’s target seats are anyway, into the real battleground. If Labour can gain between 150 and 250 new councillors there, some cautious optimism will certainly be permissible. If we wake up on 6 May to very small Labour gains in the English local elections, only registering in double figures, Labour is just treading water. If the story mostly involves Labour losses, even given Mr Khan’s likely win in London, that will be clearest warning sign yet that the worst forebodings of Mr Corbyn’s detractors may yet come true.