* New Democrats fight image that a vote for them is wasted
* Some polls show surge in popularity
* Big issue is whether Conservatives will get majority
* Liberals say only they can replace Conservatives (Adds Liberal reaction, paragraphs 13-14)
OTTAWA, April 18 (Reuters) - Canada's left-leaning New Democratic Party has never been a serious contender for power in federal politics but with its numbers jumping in some polls it's gunning for a breakthrough in the current election campaign.
"Regardless of what other parties are telling you, you have a choice," NDP leader Jack Layton declared at a news conference in Quebec City on Monday, two weeks ahead of the May 2 federal election.
The party, which wants to boost social spending and corporate taxes, has always been hampered by the fact that Canada does not have proportional representation.
That means that coming in second or third in any electoral district does not bring the NDP closer to power, so a vote for for it is often viewed as wasted. In the 2008 election, it won 18.2 percent of the vote but only 12 percent of the seats.
That's because he is fighting sometimes as many as three other parties in the fragmented field to the left of Prime Minister Stephen Harper's Conservatives.
The Liberals, the only party besides the Conservatives to have governed Canada, have regularly tried to woo NDP voters, especially at the last minute, by saying that voting Liberal is the only way to block a Conservative majority in the House of Commons.
But some recent polls put the NDP ahead of the Conservatives and the Liberals in the large province of Quebec, and an online Angus-Reid survey released on Monday even had the party tied with the Liberals nationally at 25 percent.
"People realize that Ottawa is broken, that it's not working for them," Layton said. "They're seeing the same old parties, and they're fed up, a lot of them."
Layton's performance in televised leaders' debates last week has bolstered his position, though pollsters are certainly not unanimous in showing the NDP neck and neck with the Liberals.
A Nanos rolling phone poll on Monday put the Liberals at 29.8 percent -- 10 points behind the Conservatives but more than 12 points ahead of the NDP.
Harper won minority governments in 2006 and 2008 and is now pushing hard for a majority, saying it is the only way to prevent constant elections and a lurch to the left at a time of economic fragility.
The other parties say Harper is unscrupulous in his hunger for power and must be stopped.
But Liberal leader Michael Ignatieff went further on Monday, saying that it is not simply a question of preventing a Conservative majority but replacing the Tories. He said the Liberals are the only party that can do that.
"The vote on the 2nd of May has always been a choice between governments. It's not a choice between opposition parties -- who can limit the damage, who can send a message to Ottawa," Ignatieff told a news conference in the Northwest Territories. "The choice must be who can replace Mr. Harper."
The Conservatives are polling a comfortable eight to 12 percentage points ahead of their rivals, but are projected at current levels still to fall short of a majority.
Harper pledges to pursue sustained growth and jobs without raising taxes. The Conservatives also want to go further in cracking down on crime and spending on the military.
The Liberals would roll back some corporate tax cuts and boost spending on higher education, on helping people care for sick family members and on improved pensions.
The NDP platform would roll back corporate cuts even further than the Liberals, and encourage job creation with tax credits to small businesses. It would also hire more doctors and nurses and buy ships for the navy instead of jets for the air force. (Editing by Peter Galloway)