Immigration levels and how to deal with a labour shortage facing the province will be the defining ballot question come election day Oct. 1, Philippe Couillard said Monday.

On Day 19 of the campaign, the Liberal leader keyed in on an issue that has generated significant sparring among the three main parties.

Couillard's Liberals have said they want to maintain current yearly immigration numbers of about 50,000 and will "eventually" increase that number.

The Coalition Avenir Quebec and the Parti Quebecois have both said they plan to cut the number of newcomers should they come to power.

Couillard said the question is a key one at the ballot box, with the province's long-term economic survival at stake as it looks to fill 1.3 million jobs in the next decade.

He accused his rivals of acting contrary to Quebec's economic interests as he announced $135 million in proposed spending to improve the French-language skills of immigrants and ensure they settle outside Montreal, where the lack of manpower is more pronounced.

"One of the most fundamental components of what makes an economy grow is the number of people you have to work," Couillard said. "Each time you remove one immigrant, it's not only that person you remove from the workforce, it's their children and their children's children."

The Coalition's Francois Legault has proposed reducing yearly immigration levels to 40,000 from 50,000 and his government would force new immigrants to leave the province if they don't learn French within three years.

The PQ has said it wants to reduce the number of newcomers to between 35,000 and 40,000.

In 2017, the province welcomed 52,400 newcomers.

Legault has argued that more than half of immigrants who arrive in Quebec don't speak French and end up moving or face high levels of unemployment. He has warned the use of French will gradually disappear under present conditions.

He shot back at Couillard on Monday, saying voters should ask themselves if it's time to try something new after nearly 15 years with the Liberals.

"The ballot question on Oct.1 will be about immigration, the economy, about education, and about health care," Legault said in Delson, near Montreal. "But above all, it will be about whether Quebecers want to do another four years with the Liberals."

Couillard said Quebec's retention rate is no different from that in other provinces and said his opponents are using language fear to divide.

"There is no evidence at all that French is going backwards," Couillard said. "It will always be under pressure in North America, always, but it's not true it's going backwards, it's going in the right direction."

PQ Leader Jean-Francois Lisee said the right number for him will be the one that sees new arrivals stay.

While announcing environmental-related commitments in Saguenay, Lisee accused the Liberal leader of dramatizing legitimate societal debates like immigration.

"The citizens will decide on the ballot question," Lisee said.

Elsewhere, Legault announced a Coalition government would move to reform the province's electoral system to introduce some form of proportional representation for the next provincial vote in 2022.

While Legault believes such a scenario would require the support of only one-half of the members of the national assembly, Couillard said it would necessitate unanimity.

-- with files from Julien Arsenault in Quebec City, Vicky Fragasso-Marquis in Chateauguay, Que., and Melanie Marquis in Saguenay, Que.