The minority government will need the NDP's support for its upcoming budget, which the Conservatives have already vowed to defeat, and the Liberals had better address the scheduled HST exemption for business inputs in the fiscal plan, warned Horwath.
"We want to see an overall balanced approached to balancing the books, so the corporate tax loopholes is a way to that balance," said Horwath.
"But if we see all kinds of things that are going to make life more difficult for everyday families, we are not going to be able to support that budget."
Big corporations and banks were excluded from an exemption on business inputs in the original 2009 agreement with the federal government to implement the HST, but are scheduled to start benefiting from the tax break in 2014 on entertainment, meals, telecommunications and company vehicles.
The Liberals seem only too willing to impose some kind of new tax to pay for public transit, but won't block the HST exemption for business, complained Horwath.
"You're asking a single mom who struggles every day to dig deeper into her pocket while you're telling corporations, that have been doing very well by this government, that they can get another tax break that's going to add up to $1.3 billion," she said.
"It's not balanced and it's not fair, and we've said all along we want to see a balanced and fair approach to the budget."
Premier Kathleen Wynne said she was surprised the NDP aren't on side with her in looking for new revenue tools to pay for public transit, and agreed the Liberals would consider the NDP's list of desired tax changes before the budget is introduced.
"We need to look at those," Wynne told the legislature.
"But that does not negate the need to have a serious discussion about finding revenue streams and building transit. We cannot continue to deal with the loss of productivity that is engendered by the congestion and the near-gridlock that we're confronting."
The Liberals have already reduced Ontario's corporate tax rate from 14 per cent to 11.5 per cent, but had to back off a scheduled cut to 10 per cent in order to get NDP support for last year's provincial budget. They also had to agree to an NDP demand for a new tax on incomes over $500,000.
The government is mistaken if it thinks corporations are using their tax breaks to help create jobs, said Horwath, who pointed to reports Canadian banks have been contracting out Ontario jobs to foreign workers.
"Does the premier think it makes sense to ask a working mom to pay more while telling corporations like the Royal Bank of Canada that they actually get a break," she asked.
"The government's giveaways will help the Royal Bank of Canada, who will be getting a massive tax break while they outsource Canadian jobs."
However, Wynne wasn't backing down on the need for new revenue tools to generate $2 billion a year to fund public transit improvements in the Toronto-Hamilton area.
"I think what doesn't make sense is to ask the working mum to commute for three hours from Scarborough to downtown to her job, or to drop off her child at daycare, and not have a decent way of getting to her workplace and getting home," Wynne told Horwath.
"That's what doesn't make sense."
In addition to the corporate tax changes, the NDP also want the budget to include a 15 per cent cut in auto insurance premiums, a five-day guarantee for home care services for people leaving hospitals and a new jobs program for young people.
The Liberals have not yet set a date for the provincial budget.