The escalating scandal over the C$900 million ($671 million) contract granted to WE Charity Canada has hurt Trudeau in the polls, with his Liberal Party slipping to a 4-point lead over the opposition Conservatives, an Abacus Data poll showed Thursday.
The Liberal-led government's popularity had soared amid Canada's response to the COVID-19 crisis.
Trudeau is scheduled to appear under oath at 3 p.m. (1900 GMT), while his chief of staff, Katie Telford, is to follow him at 4:15 p.m. The committee voted late Wednesday to extend Trudeau's testimony to three hours.
Conservative leader Andrew Scheer said a longer testimony is needed to get to the bottom of the affair. The committee alone cannot force Trudeau to stay on past his schedule time.
"One hour is not sufficient when we're talking about a billion dollars," he told reporters on Thursday.
Testimony by a Canadian prime minister before a parliamentary committee is very unusual. It last happened in 2006 when then-Prime Minister Stephen Harper appeared at one to speak about Senate reform.
Trudeau's testimony comes after he was put under investigation for possible conflict-of-interest violations. It is the third ethics probe he has faced in three years.
The prime minister has already apologized publicly for participating in the Cabinet decision to pick WE Charity to administer the grant program. The charity backed out shortly after the program was announced.
Canada's ethics commissioner launched the investigation after WE Charity disclosed that it had paid Trudeau's mother and brother for speaking appearances.Both the prime minister and his wife, Sophie Gregoire Trudeau, have regularly participated in WE Charity events.