But Mr. Trudeau’s problem is not, ultimately, the relative strength of his opponent. It’s that after four years of low-key success, he may not have given enough Canadians, across enough provinces, enough of a reason to give him another term.

Canada, with a land mass slightly bigger than the United States but a population of only 37 million , is a hard country to govern. Quebec, Mr. Trudeau’s home province, is a distinct society in all but name. Ontario was once an industrial powerhouse but is struggling to emerge from rust-belt status.

The Maritime Provinces, along the Atlantic, are in long-term decline, while rising Western Canada chafes against the centripetal force of Ontario and Quebec. Mr. Trudeau’s government is weighted toward those traditionally dominant central Canadian provinces, even if it doesn’t think of itself in those terms.

The result is the kind of brokerage politics that manages some people’s resentments while dissatisfying many others. Take climate change or, specifically, the balance between energy production — the West’s mainstay — and environmental protection.

Amid weak global energy prices and weak private-sector investment, the government bought a pipeline and pipeline expansion project to ship Alberta oil to the Pacific coast and on to Asia, where markets are better for producers than in the United States. But this has won few plaudits in the oil patch, which contends that the government’s environmental sensitivity caused the uncertainty that made nationalization necessary.

Mr. Trudeau has tried to assuage his more liberal base of his environmental bona fides by committing Canada to the Paris climate accords, but they complain that his ensuing carbon tax is set too low to make a difference, even as conservative provincial governments are challenging the very idea of the tax in court on the ground that it treads on provincial jurisdiction. This imbroglio may well cost Mr. Trudeau some of the few seats he holds in Western Canada, to the benefit of his Conservative opponents.

Canadian elections still turn on Quebec, where Mr. Trudeau appears dominant, and especially Ontario, where Mr. Trudeau is at the very least competitive with the Conservatives. But even there, Mr. Trudeau may be weakened by the left. The Green Party has made inroads with the sort of liberal urban voters who should be part of his base.