NDP MP Guy Caron is challenging Lise St-Denis to resign her seat and run in a byelection following her decision to leave the NDP to join the Liberals.

St-Denis made the announcement Tuesday morning, saying it was a decision she reached after months of reflection.

"The choice that I'm making is motivated by the challenges faced by my riding," said St-Denis, who represents Saint-Maurice-Champlain, former prime minister Jean Chrétien's riding. 

At a news conference following St-Denis's move, Caron said the NDP has campaigned on a policy of having floor-crossing MPs resign their seats and run again under their new party's banner. 

Voters cast their ballots in the last election to change politics, he said, but recruiting elected members from other parties is the old way of doing politics.

"Changing political affiliation is a blatant lack of respect for democracy. It encourages cynicism toward politicians," Caron said.

"Quebec voters rejected the Liberals and voted for the NDP."

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"Honestly, if the Liberals really feel that strongly about what she can contribute, we challenge them to launch a byelection where she will be able to bring the Liberal ideas, will be able to bring the values and put the work of the NDP to the test of the electors."

'Wasn't named critic'

St-Denis said her decision to jump was based on the Liberals' social policy and job-creation strategies as, as well as the NDP pulling support for the mission in Libya in its final weeks.

St-Denis said she didn't believe she'd be elected when she let her name stand as an NDP candidate, but that it wasn't unusual for her to run for the party because she'd worked for it for 10 years.

But she said she's been pondering the move almost since the May 2, 2011, election.

"It’s been six months that I’ve been reflecting and discussing," she said.

"I wasn't named a critic and I was sat in the back row because of my last name, which starts with S, so I had lots of time to look and listen to everyone," St-Denis said.

Asked about voters in her riding and whether they voted for the NDP or for her, St-Denis said simply "They voted for Jack Layton, but Jack Layton is dead."

Caron says Quebecers voted for NDP leader Jack Layton, who died in August, but also for all the values the party represents.

He said St-Denis wasn't one of the strongest members of the NDP caucus. He said the vast majority of the NDP caucus adapted well to political life, but she wasn't one who did.

"We’ll leave it for the Liberals to find out," he said, adding he's been impressed with the strength of the NDP caucus.

Caron named a number of NDP MPs who have had strong performances in the House and in committees, including Alexandre Boulerice, Hélène Laverdière and Hoang Mai.

"To be frank, I wouldn’t see her in that list," Caron said.

St-Denis had supported Thomas Mulcair for leader of the NDP. Mulcair is a former Quebec Liberal member of the national assembly.

Liberals looking to renew

Liberals are gathering in Ottawa this weekend for their biennial convention, with the need for the party to renew itself after a dismal showing in last May's general election weighing heavily on delegates' minds.

Key to the party's renewal would be a resurgence in Quebec, where turbulent polls and shifting allegiances present opportunities for political growth in now wide-open contests.

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MP Lise St-Denis is defecting from the NDP to join the Liberal Party. (House of Commons)

Interim Liberal Leader Bob Rae said he was surprised when St-Denis approached him in December to talk about joining the party. He said the political scene in Quebec is in a time of "great fluidity."

"What Madame St-Denis is reflecting is the extent of that change," Rae said.

New Democrats made historic gains in Quebec on May 2, electing 59 MPs in the province on the way to becoming the Official Opposition for the first time in party history, displacing the Liberals. The NDP's challenge now is to consolidate and build on those gains after the death of former leader Layton, whose popularity in Quebec was seen as key to the 2011 breakthrough.