NDP Leader Rachel Notley and Liberal Leader David Swann said Prentice's reversal on charitable tax credits earlier this week reinforced what had been obvious to them from the start.
"(The budget) is a platform. It's absolutely a platform," said Notley, adding, "(Former PC premier) Alison Redford ran on one plan and delivered a very different one.
"That's the record of this PC government."
Swann said Prentice is reshaping policy to deal with popularity polls that suggest a PC cake walk has turned into a three-party fight between the Tories, the Wildrose and the NDP.
"All of this (budget) is a work in progress for the premier," said Swann. "Anything that doesn't fly he just dumps.
"He's very consistent with the other four (PC) premiers in the last five years. They will do what they need to do to stay in power."
Prentice and his campaign team declined comment.
The premier called the election for May 5, a year earlier than directed by law, and he will face the other leaders Thursday in a televised debate.
Prentice said he needed a mandate to implement a long-term financial plan to steer Alberta out of the rocky shoals of low oil prices to calmer waters, where day-to-day spending would not longer be tied to swings in the resource market.
He tabled his budget in the legislature on March 26, but it has yet to be debated or passed.
The budget hikes taxes almost across the board except for corporations while cutting or freezing departmental spending and delivering a $5-billion deficit this year.
It has been applauded by business leaders and some mayors as tough but balanced. However, it has been criticized by teachers, school trustees and public sector workers as unfair and harsh for a rapidly growing province.
Prentice had originally stood firm on the plan but has now begun rolling back.
Last week, he unveiled new austerity measures, including a promise to disband 80 of the province's 320 boards, agencies and commissions by the end of the fiscal year.
On Tuesday, he shelved a planned rollback on the charity tax credit that would have added $90 million to Alberta's bottom line.
Charities said the rollback punished the needy and Prentice agreed.
Political analyst Bob Murray said it is a big gamble for Prentice.
"Sure he was able to say 'Look at the correction that we made (on charity tax credits) because we listened to Albertans,' but he's very quickly destroyed his own narrative," said Murray, vice president of research with the Frontier Centre for Public Policy.
"Now he's opened up to these accusations that he's willing to negotiate the key pieces of the fiscal plan when two weeks ago he was telling us that he's not afraid to make the tough decisions whether or not they're all that popular."
Wildrose leader Brian Jean was not available Wednesday, but party spokesman Matt Solberg said the charity tax rollback sends the message that everything surrounding the budget is built on sand.
"He said this was a budget he was proud of. This was going to be the budget," said Solberg. "Who's to say the budget after the election is going to look anything like the budget he said he needs a mandate for?"
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