The enmity between the two sides who collide again this weekend never seems to subside. Even last week, Jurgen Klopp tried to place Gary Neville's annihilation of the European Super League breakaway clubs within the narrative of Manchester United's dislike of Liverpool.
But when it comes to existential matters — preservation of these two great clubs — the relationship has always been far more complicated than that. It would perhaps be an exaggeration to describe what has existed between them at certain critical moments as 'sympathy' but there is certainly empathy and mutual understanding.
It's why some United fan groups made exploratory calls to Liverpool when the Super League plans became known last week, asking whether they might join protests at Old Trafford this weekend.
The idea of a co-ordinated protest was dropped when the ESL plans were ditched but some Liverpool fans may still make the 30-mile trip east up the M62 on Sunday.
This spirit of co-operation between the two is nothing new. United fans have never forgotten 2005. Their club had just been bought by the Glazers when they faced Arsenal in the FA Cup final and their opponents' fans mocked them mercilessly.
'Sold To The USA,' was the Arsenal reworking of Bruce Springsteen's hit that day. It did not go unnoticed among those of a United disposition that Liverpool fans in no way contributed to the ridicule.
It was soon after that Tom Hicks and George Gillett bought Liverpool, in another highly leveraged American buy-out which ultimately put the club in grave jeopardy.
'There was no point laughing at them because we knew it could happen to us. What United went through, we went through, only on a different scale,' says Jay McKenna, former leader of the highly-respected Spirit of Shankly (SoS) supporters' union which drove the campaign to oust Hicks and Gillett. 'Arsenal fans now know that, too.'
The outcome of the Hicks and Gillett effort contrasted with United's Green and Gold 2010 campaign, which fizzled out as the team remained serial winners under Sir Alex Ferguson. But it could have been a different story if Liverpool's 2005 Champions League triumph had led to further glories.
It also helped that manager Rafael Benitez publicly challenged the Americans in a way Sir Alex Ferguson never did the Glazers at Old Trafford. There was respect from the United fans for what Liverpool supporters accomplished back then.
The improbable fraternity between the two reaches back further — to the three and a half years after the Second World War when City's Maine Road was also United's home ground — Old Trafford having been bombed. For their part, City had made donations in 1902 when Newton Heath — precursor to United — were in trouble. The two clubs' directors were always close.
A month after the Munich disaster in 1958, Liverpool, then a Second Division side, offered United the services of two of their players to tide them over. In August 1971, when United fans' hooliganism caused them a two-game ban from Old Trafford and left them in need of a temporary home, Liverpool stepped up. United's only 'home' game at Anfield ended with a 3-1 win over Arsenal. The other game was played at Stoke's Victoria Ground.
But now, the battle for the soul of their clubs is one they share. The emails that representatives of SoS and the highly effective Manchester United Supporters Trust (MUST) regularly exchange, as part of the Football Supporters' Association Premier League working group, is only a part of the collaboration.
The Liverpool contingent have arguably been more militant on some recent issues, for example leading the charge to get the Premier League's detested £14.95 pay-per-view charges scrapped when some other fan groups would have settled for a compromise. But the mood this time is far more incendiary in Manchester than Liverpool.
Liverpool's success under John W Henry's ownership has much to do with that. Henry, along with executives Tom Werner and Michael Gordon, are also visible in a way the Glazers have never been.
But while Liverpool fans have challenged FSG in a series of incidents — including 10,000 fans walking out of a Sunderland home game of 2016 over reviled season-ticket pricing — there has been no such release valve for the mounting fury felt at United over detached, absentee owners who appear utterly indifferent to them.
The ESL bombshell made their intentions crystal clear — in a way not known since the Glazers' 2009 bond document, which raised the prospect of Old Trafford being sold as the owners tried to raise £500million and cut their debt burden. As Sunday's Old Trafford protests will show, the lid has been blown off resentments which have been festering for months of lockdown. United are a club who have not signed a regular first-team player since Bruno Fernandes, 18 months ago.
'This is the moment that all supporters' worst fears about the owners have been crystalised,' says one senior source at the United end. 'Things they might have got away with before the ESL will not be allowed to pass now.'
Though both clubs' blindness to how fans would respond has been astonishing, Liverpool can argue far more convincingly that they and the owners are two very different entities.
The season-ticket furore brought much soul-searching and an examination of how the club could better connect. The arrival of former journalist Tony Barrett as head of club and supporter engagement, in the aftermath of the walkout, has made a big difference — rebuilding connections and demonstrating an innate understanding of what matters to real fans.
Help for Sean Cox, attacked before a Champions League game three years ago, and more recently Mike Kearney, a fan who is registered blind, have shown Liverpool to be striking a balance between global and local.
All that groundwork made the attachment to ESL all the more egregious to fans who were told 'This Means More'. If crowds were attending football matches, the protest would have been loud and long at Liverpool's match with Newcastle United last weekend.
Yet a letter from chief executive Billy Hogan to SoS this week, apologising and agreeing to an emergency meeting, has received a cautiously positive response. Contrast that with MUST's open letter to Joel Glazer on Saturday in which they categorically refused to accept his expression of regret.
'Joel Glazer's... apology is not accepted,' the letter declared. 'Actions speak louder than words and he and his family have shown time and again that their sole motivation is personal profit at the expense of our football club. We are disgusted, embarrassed and angry at the owner's actions.'
One widely expected consequence of the breakaway plans is owners of the six British clubs making a splash in this summer's transfer market, to curry favour. But the United and Liverpool fan groups both see a far bigger picture than that. Both now want full representation and a genuine voice.
'We have a huge understanding of each other,' says McKenna. 'The reason why there's such rivalry between us is that we have so much in common.'