The Conservative leader took note Monday of the economic issues that have hurt Canada in the first three months of the year, particularly the falling price of oil.
He said the situation underscores the need to stick with his party's plan to balance the budget while lowering taxes.
"Look at Justin Trudeau over the past six weeks," Harper told a crowd of Tory faithful gathered at an Ottawa hotel.
"He's gone from saying, 'Well, deficits don't matter because the budget will balance itself,' to 'Look, we're going to have a deficit, isn't it an awful thing?' and now, 'No, I want to run deficits because they're a good thing.'
"We cannot afford people who are not ready changing their plan every couple of weeks.''
The Conservatives are gearing up for what could be a tough day Tuesday when new economic data is released that could show the economy was in a technical recession for the first half of this year.
Many economists have debated whether Canada has truly fallen into a recession, because the country has lacked some of the classic hallmarks of such an economic decline.
Harper was asked for his definition of a "technical recession,'' but he said he wouldn't weight into that debate and instead reaffirmed his position that the federal government is ahead of its fiscal projections.
"The financial position of the country remains in very strong shape,'' he said.
Also on HuffPost