With weekend negotiations failing to ease the NDP's concerns, Horwath issued a statement Sunday night urging McGuinty to take her party's issues with the budget seriously.
Horwath's chief of staff and veteran New Democrat MPP Gilles Bisson, the party's house leader, held negotiations Saturday and Sunday with their Liberal counterparts.
"Although we're pleased that discussions have been ongoing throughout the weekend, the premier must improve the budget on issues of fairness, health care, and jobs," Horwath said in her statement.
"We put forward a number of responsible proposals aimed at making the budget fairer. I'm urging the premier to take seriously these key priorities."
The Liberals said they wanted to keep the negotiations in private _ declining even to confirm who represented them in the weekend talks _ and suggested any agreement would not come before the meeting between the two party leaders.
"We've had a great weekend of discussions and those talks are ongoing, with the premier meeting Ms. Horwath on Monday," said a statement from Neala Barton, the premier's press secretary.
"Both parties have agreed that the best chance of success is to allow those discussions to take place privately. We will continue to respect that agreement."
Last week, Horwath dropped a key NDP demand to have the eight per cent provincial portion of the HST removed from home heating bills, but she still wants a two percentage point surtax on incomes the highest income earners.
"Our plan for a modest tax increase on those that earn more than $500,000 would generate the revenue needed to protect services like health care," she said in Sunday's statement.
"It's only fair that those most able to help fund our public health care system be asked to do a little more."
The New Democrats also want the government to increase payments to people on disability support, introduce a $250-million job creation tax credit, drop plans to sell off Ontario Northland railway, increase funding for daycare, home care and community care, and provide help for sectors hurt in the budget such as horse racing and tourism.
On Saturday, Horwath told thousands of protesters gathered on the front lawn of the Ontario legislature she would rather avoid triggering another election by defeating the budget, but vowed to keep pushing to change what she called a "profoundly flawed" fiscal plan from the Liberals.
"We're showing the people of this province that we are prepared to work to make the minority government work, but we're also showing them the sort of Ontario we can build together," she told the crowd.
The Progressive Conservatives vow to vote against the budget on Tuesday, so the Liberals need NDP support to avoid their defeat, which would automatically trigger another election.
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