A legislator quit Canada's main opposition New Democrats on Tuesday, adding to the problems of a left-leaning party that has failed to impress since a record-breaking performance in the May 2011 election.

Lise St-Denis, a parliamentarian from Quebec, said she was defecting to the centrist Liberals because she preferred the Liberal positions on the economy and the environment.

I couldn't sit there for three years and listen to options I didn't agree with, St-Denis told a news conference.

The Liberals slumped to a dismal third place in last year's election, losing ground to the New Democrats across the country. That split in the left-of center vote allowed the right-leaning Conservatives to win the election with a majority that guarantees they will stay in power until October 2015.

Political commentators have long argued that the New Democrats and Liberals need to merge to have a chance of beating the Conservatives. But interim Liberal leader Bob Rae said St-Denis's defection showed why such talk was problematic.

It shows you have a choice between a party which has values as well as a pragmatic approach ... and a party which is more ideological, he told the news conference.

Inspired by former leader Jack Layton, the New Democrats trounced the Liberals in the May 2011 election to become the largest opposition party for the first time, helped by a breakthrough in French-speaking province Quebec.

But Layton died in August, triggering a drawn-out leadership campaign which will not finish until late March. Interim NDP leader Nicole Turmel has struggled to make an impact, allowing Rae to become the most impressive opposition figure.

Asked if people had voted for her or for Layton, St-Denis replied bluntly: They voted for Jack Layton. Jack Layton is dead.

Turmel was not immediately available for comment. An NDP official told Reuters that St-Denis should resign her seat and fight an ensuing election as a Liberal. She told reporters she would not do so.