The old Labour Party was always a sociological alliance between, in the main, manual workers and middle class state-orientated Fabian social reformers. Both elements contained their 'Left' and 'Right', so old Labour produced Bessie Braddock and Dennis Skinner, as well as Anthony Crosland and Tony Benn. But time, and Thatcher, destroyed this sociological base, or at least decreased its overall weight in the population so it became incapable of projecting itself as a 'national' party. As a consequence it turned in on itself in a fratricidal civil war which took up much of the 1980s. Even those few shoots of modernity that appeared on the Left during this time - Livingstone and the GLC foremost amongst them - got smothered, or at least muffled, by inescapably getting caught up in this internal conflict. Kinnock stabilised the situation but little more.
Blair and New Labour took a different approach. More or less his first act was to symbolically rip up the Holy Grail - Clause 4. But this wasn't him taking sides in the old civil war, or at least that wasn't its primary meaning - it was him rejecting the whole prior sociological base of the party, and indeed its whole prior modus operandi of relating to the electorate. Henceforth New Labour was going to be about 'aspiration' and 'Middle England'. But it was going to reach out to them as a centralised marketing campaign, not grow out of them organically. Harold Wilson famously said 'the Labour Party is a crusade or it is nothing'; Tony Blair proved 'nothing' could still sell toothpaste quite successfully. (I'm using 'toothpaste as a metaphor for managerialist, technocratic policies.)
But now New Labour has run out of time, of options and, yes, out of toothpaste. It has very weak resources to fall back on: a (much smaller and sullenly depoliticised) manual working class that correctly thinks it's been ignored; the grandchildren of Fabians staffing the welfare state who feel alienated and attacked by the marketised version of 'public sector reform'; Middle England who feels its 'aspirations' aren't being met.
The historical conditions which gave birth to and sustained the Labour Party no longer pertain. It only has a ghostly after-life as a sort of zombie version of the American Democratic Party to look forward too.... A general election defeat of the scale of 1931 is not impossible.