Canadians begin this week with elections to fill all 308 seats in the House of Commons. That main branch of Parliament in Ottawa, an Ontario city that is the national capital, later selects a prime minister to become the head of Canada’s government. That choice depends on which political party wins the most seats in Monday’s national balloting.

This system of government, also used by England and now being introduced in Iraq, is called a parliamentary democracy. Canadian candidates from three main parties and several smaller ones are running in House districts known as ridings. Canada’s Parliament also has a national Senate with 105 appointed members.

The timing of this election was determined by opposition parties, which last Nov. 28 passed a resolution of “no confidence” aimed at Prime Minister Paul Martin, leader of the Liberal Party and Canada’s top elected official since December 2003. Allegations of financial misdeeds by senior Liberal officials eroded public support for the party, giving rivals a chance to force Martin to schedule an early election.

The challengers: The Conservatives, led by Stephen Harper, propose a tax cut to stimulate the economy. The New Democratic Party (NDP), led by Jack Layton, opposes large tax cuts and argues that the government instead should use part of its $9-billion budget surplus to improve social programs. Voters in Quebec also can support candidates from the Bloc Québécois, a regional party in that French-speaking province. Lastly, this could be the first time the Green Party of Canada wins seats in Parliament. It supports lower taxes for small businesses and families and higher levies for companies that consume natural resources and cause water, air or soil pollution.

The stakes: A change in government could push Canada’s public health-care system toward a two-tier system with a partial shift to non-government providers, as some Conservatives endorse. Other issues include national gun registration, same-sex marriage, U.S. trade relations and whether to continue full participation in the North American ballistic missile defense system.

The outlook: The Liberals have led Canada for 13 years, winning four elections since 1993. But opposition parties grabbed enough seats in 2003 to leave Prime Minister Martin dependent on a bipartisan coalition – an outcome that is possible again, with either a Conservative-led or Liberal-led government relying on support from other parties.