Ed Miliband is appealing to Liberal Democrat peers to join with Labour to wreck the Government's controversial NHS reforms.
The Health and Social Care Bill is set to resume its troubled parliamentary passage in the House of Lords, amid growing calls for a major overhaul.
Despite widespread opposition within the medical profession, David Cameron has given his full backing to Health Secretary Andrew Lansley.
However Lib Dem peers have tabled a series of detailed amendments while the party's president Tim Farron is urging them to strip out all new competition elements from the legislation.
The attempt to "chop the Bill" by removing the third part of it - which encourages competition - had been mooted as a compromise. But the government has rejected the idea, saying it's integrated into the rest of the Bill.
Some of the closest debates and votes look set to take place on Wednesday night, when Labour will attempt to make significant changes to the Bill.
Writing in the Sunday Mirror, Mr Miliband said Lib Dems should now combine with Labour in the upper chamber to deal a death blow to the bill.
With the Lib Dem leadership facing a potential grassroots revolt over the issue at their spring conference next month, the Labour leader warned they could suffer an even bigger backlash than they did over student tuition fees.
"This week is the time for everybody to stand up and be counted. The Liberal Democrats in the House of Lords must join with Labour to hole David Cameron's health plans below the water line," Mr Miliband said.
"The House of Lords has the chance to puncture the arrogance of an out-of-touch Prime Minister who thinks he knows better than patients, nurses and doctors and persuade him to drop this bill.
"If they do not the betrayal by the Lib Dems in allowing this bill through will be bigger than the row over university tuition fees.
"They will betray not only the people who rely on today's NHS, but also generations to come."
Milband also launched a personal attack on David Cameron, who he accused of trying to "con" people about his intentions for the NHS:
"Before the election, David Cameron promised the country he was a different type of Tory. The NHS was central to that promise.
"But he didn't tell you at the election that within months of coming to power, he would order the biggest ever top-down reorganisation of the NHS.
"He didn't tell you that he would divert billions of pounds to his bureaucratic shake-up and away from patient care. He didn't tell you that he would bring in new rules which say up to 49% of the beds in an NHS hospital could be set aside for private patients.
"But this is what he is doing. He didn't tell you because he wanted to con people into thinking the NHS was safe in his hands. The reality is on his watch, the NHS is getting worse."