(Adds White House spokesman, Trudeau meeting with lumber union)
By David Ljunggren
OTTAWA, May 8 (Reuters) - Canada escalated a trade dispute with United States by making threats Washington called inappropriate in part because Prime Minister Justin Trudeau is under pressure to secure support in a key region ahead of the country's 2019 elections.
Washington last month slapped tariffs on timber imports, prompting Trudeau to say he was considering a ban on exports of U.S. coal through Pacific ports.
As well as lumber, the administration of President Donald Trump has targeted Canadian dairy farmers, while Boeing Corp launched a trade challenge against Montreal-based planemaker Bombardier Inc.
All three are vital to the economy of Quebec, Canada's second most-populous province. And Quebec is seen as vital to Trudeau's hopes of maintaining a strong grip on power in a national election set for October 2019.
As contentious talks on renegotiating NAFTA draw closer, Trudeau has little choice but to defend dairy farmers and offer help to the lumber industry, even though that is likely to prompt fresh U.S. challenges.
"Quebec is the key," said one senior Liberal organizer.
The predominantly French-speaking province holds 78 of the 338 seats in the House of Commons and Liberals acknowledge they need to win extra seats there to offset expected losses elsewhere in 2019.
The challenge is that they captured 40 seats in Quebec in 2015, which was far more than expected.
The Liberals say they can take another 10 to 15 seats, but only if everything goes their way. This means showing support for the dairy industry - and its influential lobby - amid fresh attacks from Washington.
The United States has long complained about Canada's system of domestic protections for its dairy industry, which bars most imports and keeps prices high. Trump last month branded the industry "a disgrace."
The system is unpopular in large parts of Canada, where people complain about high prices for milk and cheese.
Trudeau, however, has little choice but to defend it.
Leger Marketing pollster Christian Bourque noted there are dairy farms in every part of Quebec.
"If you're seen as attacking farming and the land, it's probably easy for the farmers' union to get Quebeckers onside. You don't necessarily want to forget farmers," he said.
While observers see little risk of Trudeau being defeated outright in 2019, the danger for the Liberals is losing their majority, forcing them to rely on opposition parties to govern. This would inevitably mean political compromises and a diluted policy agenda.
The Liberals have so far tried to maintain calm as tensions ratchet up, relying on visits from cabinet ministers and to key states to press the message that trade benefits both sides.
The outreach efforts will continue, according to a source familiar with official strategy, adding that Ottawa will show its teeth where necessary.
"Do people honestly expect the Canadian government just to say 'We accept these lumber duties, we will move on and pay the price?'" asked the source, who requested anonymity given the sensitivity of the situation.
In Washington, White House spokesman Sean Spicer dismissed talk of a trade war.
"That's why we have dispute settlement mechanisms to do this in a responsible way," he told reporters on Monday.
In a sign of the mounting pressures on Trudeau over lumber, former Quebec Liberal premier Jean Charest said Ottawa should consider loan guarantees to affected firms.
"It is very black and white now: either the government supports them or they will just close down," he said in an interview.
Although giving such aid could prompt fresh U.S. challenges, insiders make clear Canada has no option.
Trudeau last week met with Quebec's timber unions and tweeted "supporting softwood lumber producers in Quebec and across the country is a priority."
In the short term, he faces few immediate threats. Polls show the Liberals well ahead of the opposition Conservatives and New Democrats, both of which have stand-in leaders and will not choose permanent replacements until later this year.
"He's had an exceptionally long honeymoon, he's still having a honeymoon, but that has a lot to do with the absence of opposition," said pollster Nik Nanos.
Although being seen to openly favor one province or region over another can be politically fatal in Canada, Liberal sensitivity toward Quebec is clear.
When it came time to deciding on aid to Bombardier - which has received billions in subsidies from Ottawa - the Liberals made clear the only question was not if, but how much.
Party operatives also admitted relief once became clear Ottawa would not have to decide before the election on whether to allow TransCanada Corp to build an oil pipeline across Quebec.
Environmentalists and aboriginal activists had promised protests that Quebec Liberals said they feared could hurt the party's chances. (Additional reporting by Andrea Hopkins, editing by G Crosse)