TORONTO -- Ontario's NDP says fundraising and campaign finance reforms proposed by the Liberals will hamper the ability of interest groups to make their voices heard during elections, while giving the government a free pass on advertising.

The Liberal government drafted legislation and invited the opposition parties to meet with them last week to provide input, but the NDP refused to attend, calling the consultation a sham.

Party leader Andrea Horwath released a letter Thursday saying the NDP supports a ban on corporate and union donations, and is "broadly supportive" of restricting third-party advertising.

However, she wrote, restricting third parties to spending $100,000 in advertising during an election and having no corresponding restriction on government advertising gives the government an unfair advantage.

"You cannot proceed with legislation that severely restricts the ability of non-partisan organizations and opposition parties to communicate with the public while maintaining the ability of the Liberal government to spend unlimited amounts of public money and use the full resources of government to communicate its position on any issue," Horwath wrote.

The draft legislation proposes changing the definition of political advertising from promoting or opposing any party or candidate to taking a position "on an issue with which a registered party or candidate is associated."

That captures "virtually every issue of public interest," Horwath wrote.

Current rules say the government can't advertise during an election, unless it relates to a revenue-generating activity, is time sensitive or "meets any other criteria that may be prescribed."

Deputy Premier Deb Matthews, when asked, had not yet seen Horwath's letter, but said having limits on third-party advertising is important.

"We see in the States what can happen if advertising's unrestricted," she said. "It can totally distort that election campaign."

Parties can spend 80 cents per voter per riding where they run a candidate and candidates can spend $1.28 per voter in the riding. The government is proposing adding a spending limit of $1 million during the six months before the start of a general election. Ontario's current rules require third parties to register and report their advertising expenditures during elections, but there are no limits on what they can spend.

Advertising by third parties in the 2014 general election topped $8 million.

The Liberals are also proposing to lower donation caps to $1,550 to a political party and another $3,100 each to candidates and constituency associations during an election period. That would mean that people could donate up to $7,750 to a party and its candidates during a year with a general election. In a year with, for example, two byelections, that limit would be $10,850.

The NDP want a $3,100 annual hard cap, in total for contributions to a party and a candidate or constituency association.