Ed Miliband urged MPs to "stand up for the victims" of press abuse by enshrining a new press regulator in law in a crunch Commons vote.
"Monday is the day that politics has got to do the duty by the victims and has got to stand up for the victims," he told The Observer.
"I think it is an important moment because we have had decades of failing to ensure that we have a system of press complaints and redress which means that ordinary people aren't left at the whim of a sometimes abusive press."
Rival proposals for a new system of newspaper self-regulation will face judgment tomorrow in a high-stakes battle over the response to the phone-hacking scandal.
In a reversal of the Westminster status quo, Labour and the Liberal Democrats are lining up against the Conservatives in a coalition for a tougher regime.
Both sides propose using a royal charter to create a new watchdog body in response to the recommendations of the Leveson Inquiry into abuses by the press.
But the Lib/Lab version involves underpinning the system in legislation, something which has been opposed by Prime Minister David Cameron as a risk to press freedom.
It also rules out a veto for the press over the membership of the watchdog and gives the regulator the power to enforce the publication of prominent apologies.
Cameron - who dramatically pulled the plug on cross-party talks on Thursday - has said he will abide by the will of parliament.
Without an overall Commons majority, allies accept he is unlikely to win sufficient support for using the threat of punitive damages against non-participants to persuade papers to sign up.
Despite efforts to shore up the vote - including bringing a minister back from Japan - a number of Tories are expected to back the Lib/Lab proposals.
Cameron insists his proposal is "the fastest possible way to deliver the strong self-regulation body that Leveson proposed that can put in place million-pound fines, prominent apologies and get justice for victims in this country".
However on Saturday he indicated that he did not consider the Lib/Lab proposed statutory underpinning "a big issue of principle" and said he was "confident about the future".
"We're now in sight of getting that regulator with million pound fines and proper regulation in our country," he said.
"The idea of a law, a great big all singing, all dancing media law that would have been bad for press freedom, bad for individual freedom - that's off the table."
He insisted he too was acting as "a friend of the victims" of phone hacking.
But Miliband said politicians of all colours now needed to follow their instincts about what was right.
He told the Observer: "Now we are at this moment which is a sort of crossroads: do we change or is it more of the same?
"I think we need to choose the right course, and I think it is a test of politics."
There was criticism of the failure to incorporate fully Lord Justice Leveson's proposals from the parents of murdered schoolgirl Milly Dowler.
It was the revelation that News of the World journalists had hacked Milly's phone which prompted the paper's closure and the 16-month Leveson Inquiry.
Bob and Sally Dowler said: "Given the considerable investment of time and money in the Leveson Inquiry, we are very disappointed to learn that Lord Justice Leveson's proposals may not now be taken forward if the politicians choose to ignore the recommendations of Lord Justice Leveson that were aimed at preventing the sort of abuses that we and so many others suffered," they said.
Actor Hugh Grant, who fronts the Hacked Off campaign, said Cameron faced defeat because he was "so clearly on the wrong side in this".
"When he was forced to choose between honouring his promises to the victims of years of press abuses or staying cosy with the owners of Conservative-supporting newspapers, he chose the press barons," he wrote in The Observer.
Grant said victims such as the Dowlers would support the Lib/Lab package even though a royal charter was not ideal and enacts Leveson only "reasonably well".
He insisted there were "gigantic differences" with the Prime Minister's scheme, which was "cunningly designed" to return to the previous discredited system.
"It was never ideal that this would be done by royal charter. That seemed to us strange to haul this medieval instrument out of history," he told BBC1's Andrew Marr Show.
"But is is not bad. The important victims of this - I do not include people like myself, I am talking about people like the Dowlers etc - would be supportive of it, yes.
"MPs promised victims to do right by them and they have that chance on Monday."
The actor confirmed that he personally called members of the shadow cabinet last week to warn them not to settle "much too early" amid signs of cross-party agreement.
But he dismissed the idea that Hacked Off was a "smooth sinister operation" - mocking his colleagues such as ex-MP Evan Harris and director Professor Brian Cathcart.
Grant said: "It's a few dandruffy professors sitting in a cheap office with a slightly insane, chess champion ex-Lib Dem MP and a couple of threadbare lawyers and me."
He said he expected "a number of Tory rebels".
He also said he was so confident of the weakness of the Conservative charter that if it ever levied the £1 million fines heralded by the Prime Minister, he would match it for charity.
"If their scheme is set up and there is a £1 million fine, I will match it and give it to Comic Relief. It will not happen because they have designed an investigative scheme so labyrinthine and complex," Grant added.
Chancellor George Osborne said he hoped a last-minute deal could still be found - warning a regulatory system that did not have cross-party support might not last.
"We want to make sure we have a system of press regulation that prevents the abuses we saw in the past happening again, but also makes sure we have a free press in this country," he told BBC1's Andrew Marr Show.
"That is such a precious thing: people fought - and literally died - to give us a free press.
"The Prime Minister has already achieved a huge amount in protecting a free press from those who want to undermine it.
"I would say it would be great on Monday if we can get some kind of agreement, even at this late stage, between the parties.
"Because, frankly, I think press regulation which is achieved in a way which divides the political parties is not press regulation that is really going to last, and it is not going to be a press regulation which is deeply-rooted in our culture.
"There is still an opportunity for us to get together and get a press law that works. Ultimately we are not about grandstanding on this; we are about getting a press law that works."
Shadow culture secretary Harriet Harman said Labour was talking to "many" Tory MPs it hoped to join it - but held out the prospect of Cameron avoiding defeat by doing a deal.
There were "just a few issues" remaining between the two sides - with the "substantive" ones being the powers to force prominent apologies and the press veto over membership.
"We've always said we would like to reach agreement and actually we could then come to the House of Commons with an agreed position and say 'yes, this is what we want," she told the Sky News Murnaghan programme.
"There are just a few issues that remain between us, but they are quite important ones."
The legislative underpinning was simply to ensure ministers in future could not "tamper" with the system - either watering it down or cracking down on press freedom, she suggested.
"I do not think it is a party political issue and I think there is a will of the House of Commons," she said.
"If we could reach agreement we could come to the House of Commons with an agreed package and the House of Commons could say 'yes we agree with that'.
"We've really got to make sure that we have a proper, tough regulator which has fair rules but where we don't slip back like we have always done before."
Liberal Democrat deputy leader Simon Hughes suggested between 20 and 60 Tory MPs could rebel and be joined by smaller parties - which would mean a very heavy defeat for Mr Cameron.
"On our side of the argument there are Conservatives - up to 60 at the maximum but probably 20 core people.
"There are Irish parties, nationalist parties, the Green Party, Labour, the Liberal Democrats and possibly some Tories - now that's quite a big coalition.
"I hope that what will happen tomorrow is that the will of the House will be that we implement Leveson as closely as possible, that the Prime Minister will accept that, and it will therefore be clear that the view of Parliament, across the parties after having read Lord Justice Leveson's report, is that we have a better, stronger system of dealing with the press."
Author JK Rowling - who gave evidence to Leveson about press intrusion into her family's privacy - said victims had been "hung out to dry" by Cameron and urged Labour and the Liberal Democrats to "have the courage" to protect them.
She said she would not "support anything that hampers the press's ability to hold power to account" but that the Leveson recommendations were "reasonable and proportionate".
"It goes without saying that what my family has been through - and I spent two hours re-living those experiences on the stand at the inquiry - is less than nothing compared to what was meted out to the McCanns, the Watsons and the Dowlers: ordinary families who became newsworthy through terrible personal tragedies, or to Chris Jeffries, who was literally in the wrong place at the wrong time and found his life forever changed.
"I believed David Cameron when he said that he would implement Leveson's recommendations 'unless they were bonkers'. I did not see how he could back away, with honour, from words so bold and unequivocal. Well, he has backed away, and I am one among many who feel they have been hung out to dry. Monday's vote will make history one way or another; I am merely one among many turning their eyes towards Ed Miliband and Nick Clegg and hoping that they have the courage to do what Cameron promised, but which he failed to deliver."
Press Complaints Commission chairman Lord Hunt said major publishers were ready to sign up to a tough new "Leveson compliant" watchdog but believed statutory underpinning would send a dangerous signal to the world about press freedom.
He welcomed the parties' agreement on a royal charter and noted that Lord Justice Leveson concluded that "the last thing he wanted was statutory press regulation".
"All those that I have consulted who are the publishers, they have all agreed to have a new body and a fresh start - with teeth," he told Murnaghan.
"Probably the strongest regulatory body in Europe: ability to fine; ability to carry out investigations.
"They have all signed up to that and they just say 'we don't need statute; the last thing we should do is send a message across the world that the UK now has a press law'."