"My party wants to get Mr. Harper to make some constructive changes on EI before this summer," Ignatieff said late Thursday in a speech to a labour union.
"And I'm telling you straight and I'm telling you clear: I cannot continue to make Parliament work unless we get substantive EI reform before this Parliament rises in June."
Ignatieff's comments came a day after Harper shot down his demand to set a national minimum threshold of 360 hours to collect EI benefits as a temporary measure during the recession. Currently, eligibility and benefits vary by region.
The Opposition leader urged Harper to rethink his position, and insisted that he's doing his best to make Parliament work.
"That's what Canadians are overwhelmingly saying: 'Try to make it work.' So I'm trying to make that work," Ignatieff said after the speech.
"But I can't back down on an issue as fundamental as EI. This affects hundreds of thousands of Canadians."
Harper's aides have stressed the prime minister has not entirely closed the door to further EI changes - just the type being proposed by the opposition - which they argue would increase payroll taxes.
The Liberals say the proposal would cost taxpayers $1.5 billion a year, but would only be a short-lived measure.
It marked a policy shift for the Liberals, who in the 1990s tightened EI eligibility rules to between 420 hours and 910 hours worked, depending on various factors.
However, both leaders appear to be girding for an election over EI, with Harper declaring himself ready to "take him on" over the issue.
Adding fuel to the fire are recent Conservative attack ads depicting Ignatieff as a long-absent political opportunist. Ignatieff fired back Thursday, warning that his party will soon retaliate with their own ads.
"I don't see why I need to reply in kind. Somebody's got to keep the bar up in the right place," he said.
"But we will reply. We will have vigorous ads attacking their record, you bet."
The Liberal ads will target the government's "skyrocketing" deficit and its failure to get much-needed stimulus funds out the door to kick-start the economy, he said.
"These are serious failings that the country needs to know about," he said.
Ignatieff condemned the Tories for launching the "offensive" ads while the country was suffering through its worst economic crisis in generations.
"When you're down in the polls, when you're presiding over the worst collapse in employment in recent memory, when you've got record bankruptcies everywhere you look, the thing you're going to do if you're in government and responsible for this mess - you want to change the channel, and that's what they're trying to do," he said.
Ignatieff also took aim at Harper, saying he dismisses those who live and work outside the country as "somehow less Canadian."
He says whenever Harper is in a jam, he retreats to the politics of division, pitting one group against another.
Ignatieff says while he's spent many years abroad - like many other citizens - he's still Canadian.
"Canadians who chose to work outside the country don't pretend that Canada is not their country," said Dimitri Soudas, a spokesman for Harper.
"This is not about Mr. Ignatieff being out of the country for 34 years, it has to do with the fact that he came back only to become Prime Minister and openly says he will leave again if he doesn't get his way."