MONTREAL (Reuters) - The Canadian province of Quebec holds an election on Monday that is primed to be a showdown between the long-dominant Liberals and a center-right party, with immigration playing out as a key issue.

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The election in Quebec, home to the majority of Canada’s influential dairy farmers, comes as Prime Minister Justin Trudeau’s Liberal government decided to open up the country’s dairy industry to the United States, as part of concessions made to strike a last-minute deal for a renegotiated NAFTA.

Recent opinion polls have shown Quebec’s ruling Liberals, a centrist party, running neck-and-neck against the center-right Coalition Avenir Quebec (CAQ) of former business executive Francois Legault, which has never held power.

Both provincial party leaders said on Monday they would support Quebec farmers concerned about the NAFTA concessions.

“We will look at all options to defend our farmers,” Legault told reporters.

In the deal agreed late on Sunday, Canada agreed to give U.S. dairy farmers access to about 3.5 percent of the country’s approximately $16 billion annual domestic dairy market.

A win by the CAQ on Monday would follow a shift to the right in Ontario, where Premier Doug Ford’s Progressive Conservative government swept to power in June, ending 15 years of Liberal rule in Canada’s most populous province, which includes Toronto.

A CAQ victory would also dent Trudeau’s hopes of winning re-election in October 2019, as the federal Liberals are betting on gains in Quebec to offset expected losses elsewhere then.

IMMIGRATION STANCES

The question of how many outsiders should be let into the majority French-speaking province - the second most populous in Canada - has eclipsed arguments over separatism that have dominated politics in Quebec in recent decades.

While the two business-friendly front-runners have garnered around 30 percent of the vote each in recent polls, the CAQ is expected to win more seats because it has greater support among francophone voters who are key to winning elections.

Legault has campaigned on a controversial plan to take in 10,000 fewer immigrants a year and to expel new residents who fail to pass tests on French and Quebec values within three years.

The Quebec Liberals have been in power for 13 of the last 15 years and the CAQ is running on a platform of change.

Meanwhile the Parti Quebecois, which advocates for Quebec separating from Canada, risks its worst showing as it fends off a challenge from left-leaning Quebec Solidaire, pollsters say.