OTTAWA -- Parliament reopens for business Monday after a 12-week summer break.

Here's a look at the priorities the three main political parties have staked out and the challenges each faces in what has the potential to be a tumultuous fall sitting of the House of Commons:

LIBERALS

Priorities

-- Stimulating the stalled economy and improving conditions for the middle class. That includes passing the bill formalizing a tax cut for middle income earners and the second part of the bill implementing last March's budget.

The government is also hoping the roll-out of funding for major infrastructure projects will boost the economy. It is also working on finalizing a free trade agreement with the European Union. And Prime Minister Justin Trudeau will make the case for investing in Canada to some of the most prominent international pension funds and portfolio managers at a Toronto summit on Nov. 14 organized by BlackRock, the world's largest asset manager.

-- Striking an agreement with the provinces on a national climate change strategy, which is to include putting a price on carbon. A first ministers' meeting is expected in November.

-- Finding consensus on reforming Canada's first-past-the-post voting system. An all-party committee is to report with a recommended alternative by Dec. 1. Trudeau has vowed that last fall's federal election will be the last conducted under FPTP.

-- Negotiating a new long-term health accord with the provinces that focuses on a pharmacare strategy and improving home care, palliative care and mental health services.

-- Deciding where to send Canadian peacekeepers. Trudeau has pledged to send up to 600 military personnel and 150 police officers to take part in United Nations peacekeeping missions.

Challenges

-- A stubbornly stagnant economy, causing job losses and a rising unemployment rate.

-- A host of thorny issues which require hard choices that will inevitably disappoint some segments of the population, including electoral reform, pipelines, climate change and how to proceed with the purchase of new fighter jets.

-- The potential for getting knocked off message by events, such as the expenses controversy that has plagued Health Minister Jane Philpott and several other ministers over the summer.

CONSERVATIVES

Priorities

-- Guarding taxpayers against the government's big-spending, high-deficit agenda and coming tax hikes in the form of carbon pricing and increased CPP premiums.

-- Demanding a debate and vote on whatever peacekeeping mission(s) the government ultimately chooses to participate in.

-- Demanding a national referendum on any change to the electoral system.

Challenges

-- Keeping a coherent message and a united front in the midst of a leadership contest. That could prove difficult, as has been demonstrated by the controversy over leadership contender Kellie Leitch's proposal to screen prospective immigrants for "anti-Canadian values."

NDP

Priorities

-- Pressuring the government to accept a new voting system based on proportional representation, in which a party's share of seats in the Commons would more accurately reflect its share of the popular vote.

-- Holding the government to account for worsening economic conditions, particularly job losses, record levels of family debt, seniors living in poverty and the housing affordability crisis.

-- Holding the government to account for its failure to deliver on promises to improve conditions for indigenous peoples, protect public health care and combat climate change.

Challenges

-- Like the Tories, maintaining a coherent message and united front in the midst of a leadership race.

This is a more daunting challenge for the NDP due to slumping poll numbers, plummeting donations and ongoing misgivings by some New Democrats about Tom Mulcair remaining leader until a successor is chosen in October 2017. Mulcair survived an attempted coup last week but could face more efforts to push him out if the party's fortunes continue their downward spiral.