While the federal Liberals cheer MP Lise St-Denis' decision to trade her NDP membership for a seat in the Grit caucus, New Democrats counter that the move represents a "blatant lack of respect for democracy."

Quebec MP Lise St-Denis made the announcement at a press conference in Ottawa Tuesday.

"I have decided to join the Liberal Party of Canada," St-Denis said, explaining that she came to her decision while observing the Liberal caucus at work in Ottawa over the last few weeks.

"I recognize today not only the collective value of their work, but more so their sense of duty and commitment," she said.

Crossing the floor, she added, does not represent a shift in her politics, but rather "the continuity of my thought process on Canada's future and the place that must be taken in our institutions by Quebecois and francophones all over the country."

St-Denis, who was one of 59 NDP MPs elected in the spring election, won her seat in the Saint-Maurice-Champlain riding with 39 per cent of the vote. The Liberal candidate for the riding once held by former Liberal prime minister Jean Chretien garnered just 12 per cent of the vote.

The one-time supporter of fellow Quebecer Thomas Mulcair's NDP leadership bid said her decision to switch parties "is that of a woman free to express her political opinion."

Her motivation, St-Denis explained, is rooted in a view of Canada's political future she now realizes is shared with the Liberals.

"We live in a time of fundamental transformation, globally and nationally. It is thus crucial that we make choices that will undoubtedly protect who we are and who we want to be."

On hand for the press conference, interim Liberal Leader Bob Rae was quick to welcome the newest member of his federal caucus.

"The rebuilding of the Liberal Party of Canada depends on people like Ms. St-Denis who have the courage of their convictions and who join our fight against a Conservative government rooted in rigid and dangerous ideology," he said in a statement.

The Liberals' Quebec caucus chair was similarly effusive in his statement of "heartfelt welcome."

"I have always admired people who make decisions based on their principles and convictions, and I can assure Lise, on behalf of my colleagues in our Quebec Caucus, that she will have all the support necessary to better serve the constituents of Saint-Maurice-Champlain," Denis Coderre said.

Coming the day before Liberal MPs and Senators are due to gather for their two-day policy strategy session in Ottawa Wednesday, there's little doubt the floor-crossing announcement could boost morale.

Later on Tuesday, Coderre revealed that the road to defection began in earnest several weeks ago, when St-Denis first made contact.

"She's a woman of reflection and she's doing her duty on policy," Coderre told CTV's Power Play, adding that St-Denis has sought pragmatism over dogmatism.

In December, Coderre told St-Denis to think about her decision over the holidays, and to make sure she wanted to go through with it.

Among her gripes with the NDP were policy decisions about the bombing mission in Libya and the federally aided construction of the Champlain bridge in Montreal, Coderre said.

"I think that we have to realize that we have a woman who didn't feel that she was in the right place," Coderre said.

NDP issues byelection challenge

But the NDP made it perfectly clear they were not going to let St-Denis' defection go unchallenged, and New Democrat MP Peter Stoffer said that the voters should have the final say.

"I think it's wrong, if she wants to become a Liberal member she should go back and seek a by-election," he said.

Stoffer added that Parliament will debate a bill next month that would seek to eliminate floor-crossing from the political landscape.

"The seat belongs to the people of the riding," he added.

While pundits have said the defection is a heavy blow to an NDP caucus that has appeared rudderless since the death of former leader Jack Layton last summer, Stouffer is remaining upbeat about his party's future.

He characterized the incident as a "speed bump," and he stressed that his party will remain optimistic and focused on the election of a new leader on March 24.

Earlier, NDP Quebec Caucus Chair Guy Caron was blunt in his party's response.

"Recruiting elected members from other parties is clearly the old way of politics," Caron said, characterizing the recent votes for his party as a signal Canadians "have had enough of cynicism, cronyism, separatism and partisanship.

"Changing political affiliation is a blatant lack of respect for democracy that encourages cynicism toward politicians," he continued, before issuing the Grits a challenge.

"If the Liberals think it's what the voters of the riding want, we challenge them to run Madam St-Denis in a byelection," he said.

St-Denis' defection means the third-party Liberals' seat count bumps up to 35, including 8 from Quebec. The official Opposition NDP is down to 101 seats, while the majority Conservatives are down to 165, after Peter Goldring changed his affiliation to Civil Libertarian. The Bloc's four MPs and the single seat held by Green Party Leader Elizabeth May round out the Commons seat count.

Parliament convenes for its next session on January 30.