All three candidates vying to replace Patrick Brown as Ontario Progressive Conservative leader are coming out against the carbon tax in their party's platform, leaving a $4-billion hole in their fiscal plan.

Caroline Mulroney announced she "will not support" the carbon tax Thursday morning, only three days after repeatedly defending it as a necessary evil. Christine Elliott also came out against the carbon tax Thursday, declaring that an online survey found 92 per cent opposition. Doug Ford was against it from the launch of his leadership campaign.

Brown put a carbon tax in the party platform, unveiled little more than two months ago. It is similar to the system in place in British Columbia. It would replace the cap-and-trade program introduced by Premier Kathleen Wynne's Liberal government, which requires industry to buy permits for greenhouse gas emissions.

The revenue from the carbon tax is crucial to the rest of the PC platform, generate $4 billion over its first three years. It helps pay for some of the goodies that the PCs are dangling in front of voters in hopes of winning Ontario's June 7 election, including a 22 per cent income tax cut and thousands of dollars in child-care rebates.

Mulroney stated her initial position on the carbon earlier this week in multiple interviews and during a public chat with her campaign co-chair, federal Conservative deputy leader Lisa Raitt.

Leadership hopeful Doug Ford rejects the proposed carbon tax. (Julie-Anne Lamoureux/Radio-Canada)

"It's a federal tax," Mulroney told Raitt on Monday. "At this point, we have two options. Either the federal government can retain the revenues and figure out how they're going to spend it, or we can control that. We [Progressive Conservatives] all know that we're better at making sure that we put money back in people's pockets than they would be."

Mulroney did a U-turn on Thursday morning, however, with this statement on Twitter:

As the Leader of our party I will not support a carbon tax. As Premier of Ontario I am going to explore options to oppose it. Read my statement here: <a href="https://t.co/E5GhRuh3Ny">https://t.co/E5GhRuh3Ny</a> <a href="https://twitter.com/hashtag/LetsGetItDone?src=hash&ref_src=twsrc%5Etfw">#LetsGetItDone</a> <a href="https://twitter.com/hashtag/onpoli?src=hash&ref_src=twsrc%5Etfw">#onpoli</a> —@C_Mulroney

Speaking before Mulroney shifted gears, Ford rejected her stance. "I won't kowtow to the prime minister, as she might," he said Wednesday in an interview with CBC News.

"It's just a tax grab," Ford said. "Carbon tax in my opinion is a bad tax — it's a bad tax for businesses, it's a bad tax for people."

"Just saying that you would get rid of the carbon tax is easy to say but it's neither fiscally nor environmentally responsible," Elliott said in a phone interview Wednesday.

Christine Elliot said Wednesday she wanted to hear from party members before making a stand on the carbon tax, then on Thursday came out against it. (The Canadian Press)

Elliott said she would survey party members for their view on various options and will take a stand before voting begins in the leadership race March 2.

"I've heard from many of our members that that was one aspect of the platform that they did not have any input into," said Elliott. "The members may well choose the carbon tax and I am certainly happy to run on that, but I want to know what our members think."

Then on Thursday, Elliott took her position, citing the results of an online survey she conducted:

I've been a party member for over 20 years. I've always opposed a carbon tax. I knew our members would agree — our survey confirmed that! Now, I'm ready to take the fight to Kathleen Wynne. Are you ready? <a href="https://t.co/33R4CbbOUU">pic.twitter.com/33R4CbbOUU</a> —@celliottability

Ford insists there are ways to find waste and mismanagement in the provincial government's $140 billion annual budget to make up for the lost revenue.

But he is not offering any specific proposals about how he would reduce greenhouse gas emissions.

Caroline Mulroney said Monday she'd keep the carbon tax, even though she doesn't like it, then on Thursday issued a statement saying, "As the leader of our party I will not support a carbon tax." (Aizick Grimman, Chris Mulligan/CBC)

"I'm all environmentally friendly. I'm not one of these naysayers, saying there isn't climate warming. I believe there is," Ford said. "If there was some magical way to stop emissions and stop everything, I'd sit back and say, 'Sure, let's look at it.'"

The leadership race was triggered by Brown's resignation, following accusations of sexual misconduct. Brown has denied the allegations, which have not been proven in court. The PCs will announce the winner of the leadership race on March 10.