Ontario's Liberals say the NDP made a $5.7 billion mistake in the budget of its election platform, but the NDP says the criticism is nothing more than a "pathetic attempt" to discredit its plans.

The Liberals said Monday that the NDP platform is built on the 2018-19 budget tabled in March. However, while the NDP is budgeting for $140.2 billion in program spending, the actual program spending included in the budget is $145.9 billion.

Liberal party officials said it appears the NDP subtracted $5.7 billion in new spending initiatives built into the latest budget, showing a failure of basic competency.

Liberal leader Kathleen Wynne and the Liberal candidate for Mississauga South, Charles Sousa, criticized the NDP's budget at a Toronto campaign event.

"The NDP built their platform and their entire economic plan for the province on a major mistake," Sousa said.

Charles Sousa and Kathleen Wynne teamed up to criticize the NDP's election platform budget on Monday. (CBC)

Sousa says if the NDP were to pass its plan as outlined, it would defund valuable programs — something the NDP has said it won't do.

He the challenged the NDP to fix the "undeniable" error, saying without doing so the platform is "incoherent and unrealistic."

The NDP platform does promise new spending in addition to what the Liberals tabled in their budget, but does not account for such initiatives as $300 million to double the number of guidance counsellors in elementary schools and the $800 million program to help seniors to stay in their homes longer.

In a statement, the NDP defended its budget, calling it a "fully-costed and verified plan."

The party notes its budget doesn't include all of the Liberal spending promises because the NDP disagrees with several aspects, including some tax changes and the Liberal dental care plan.

"Wynne's pathetic attempt to discredit the NDP only reminds Ontarians how badly they've been let down by her," the statement said.

Currently, CBC's Ontario poll tracker shows the NDP leading the Liberals, though both parties continue to trail the PCs.

NDP leader fires back

NDP Leader Andrea Horwath attacked the Liberals' budgeting when questioned about the spending in her party's election platform. (CBC)

NDP Leader Andrea Horwath is in Brampton on Monday morning, where she vowed to build a third hospital to serve the rapidly-growing city.

When reporters asked if her budget figures add up, Horwath attacked the Liberals.

"Anybody who thinks that the Liberals numbers are sound hasn't been paying attention to what they've been doing at Queen's Park for the last number of years," she said, noting concerns raised by Ontario's auditor general and financial accountability office.

Horwath also firmly told reporters she's not interested in forming a coalition government with the Liberals.