By Anthony Esposito and Dave Graham

MEXICO CITY, Feb 25 (Reuters) - Mexico and Canada aim to finish reworking less contentious chapters of the NAFTA trade deal with the United States in new talks beginning on Sunday, hoping to clear the path for a breakthrough on the toughest issues before upcoming elections.

In six months, negotiators have ground out progress on the technical details of a revamped North American Free Trade Agreement, but made little advance on the most radical demands made by the administration of U.S. President Donald Trump.

Ranging from demands for major changes to automotive content rules and dispute resolution mechanisms, to imposing a clause that could automatically kill NAFTA after five years, the chief stumbling blocks laid by the White House look unlikely to be removed in the latest Mexico City round, officials say.

But if the three negotiating teams manage to iron out remaining differences on areas of broader consensus, officials hope the political leaders will turn their attention to brokering a compromise on the trickiest U.S. proposals.

"I think there's going to be major progress on the technical issues and major obstacles on the critical issues," Bosco de la Vega, head of Mexican farm lobby the National Agricultural Council, said of the talks running until March 5.

Once agreement is reached on technical chapters such as state-owned enterprises, barriers to trade and e-commerce, about 10 percent of the modernized accord would eventually be left over for political leaders to work out, de la Vega estimated.

Mexico hopes to build on the previous round of talks in Montreal, when Canada floated what it called "creative" solutions to the U.S. demands.

"We will address and deepen the discussion on Canada's proposals," a Mexican official told Reuters, speaking on condition of anonymity due to the sensitivity of the matter.

Canada countered U.S. demands for a higher auto content requirement with a proposal to include expenses for engineering, research and development and other high-value work in the total.

Under NAFTA, at least 62.5 percent of the net cost of a passenger car or light truck must originate in the region to avoid tariffs. Trump wants the threshold raised to 85 percent.

"You can't have a successful negotiation if there's no change to the rules of origin," said the Mexican official, adding, "it won't be 85 percent. We're not sure what the number is going to be."

Mexico's Economy Minister Ildefonso Guajardo has said his negotiating team aims to present a proposal on rules of origin, though he has not provided details.