* reuters://realtime/verb=Open/url=cpurl://apps.cp./Apps/fx-polls?RIC=BRL= LatAm FX poll data
SAO PAULO, Jan 5 (Reuters) - As Latin America braces for potentially game-changing presidential elections this year, a Reuters poll of currency strategists and economists revealed diverging outlooks for the region's two main currencies.
The Mexican peso is forecast to weaken in the first half, as traders hedge against a potential populist victory, before soaring by year-end as it becomes clearer that even a populist president will struggle to pass radical measures in Congress.
In contrast, the Brazilian real will probably slip steadily throughout the year, though uncertainty over exactly how much reached a six-month high.
Regional drivers are likely to matter most in 2018, forecasters said, even as global central banks tighten policy and geopolitical risks simmer.
The peso is set to weaken 2 percent in six months to 19.75 per dollar, according to the median of 18 forecasts taken Jan 3-4. It is then forecast to surge 4 percent to 18.6 by year-end.
That is a more pronounced move than the one forecast in the December poll, which showed the peso weakening 2.3 percent in six months before turning flat in a year.
The boomerang motion suggests concerns over a substantial economic hit stemming from trade negotiations between Mexico, the United States and Canada, which drove the peso to all-time lows in 2017, are unlikely to materialize.
Grupo Banorte expects talks to culminate in a favorable agreement but acknowledged that "tensions between the negotiating teams will increase further before they wane."
"By the first quarter, we're likely to be clueless as to what is going to happen," said Alejandro Padilla, Banorte's head strategist for fixed-income and FX.
Several investors have expressed concern that Andrés Manuel López Obrador, who has been leading public opinion polls, is pursuing a nationalist platform and will stoke tensions with U.S. President Donald Trump's administration.
Should he seem likely to win, traders will react by seeking protection in the U.S. dollar, weighing on the peso, forecasters said.
In past elections, the Mexican currency weakened as much as two standard deviations three months before the vote, Padilla said. That would entail a depreciation of around 1.4 peso, according to Reuters calculations, or more than 7 percent over the current rate.
Still, Padilla expects electoral tensions to ease as the year advances.
"Regardless of who wins the 2018 elections, we should observe a relief rally in the peso as investors will acknowledge that macroeconomic stability will prevail given the strength of institutions and the counterweight of Congress," he said.
Central bank efforts to fight inflation by keeping interest rates high should also shield the peso, even if Trump's overhaul of the United States' tax code stokes inflation and forces the Federal Reserve to raise rates, he added.
In contrast, forecasters diverged widely on whether Brazilian President Michel Temer's platform of structural reforms will survive the most wide-open and difficult to predict election in decades.
The Brazilian real is seen weakening 2.3 percent in six months and 3.4 percent by year-end, according to the median of 40 estimates, compared to previous forecasts of 2 percent and 2.6 percent.
But the standard deviation for 12-month estimates, a measure of dispersion, reached the highest level since the July poll, after a corruption scandal had all but derailed Temer's pension reform plan.
"It would be ludicrous to expect 2018 to be a calm year," said André Freitas, a manager at Verus Gestão de Patrimônio.
Former President Luiz Inácio Lula da Silva, who has rallied against austerity, is leading polls, but he could be barred from running if his conviction for corruption is upheld on Jan. 24.
Otherwise, the field is split between various potential candidates from law-and-order congressman Jair Bolsonaro and businessman-turned-mayor João Doria to nationalist Ciro Gomes.
Freitas expects the nation's recovery from the deepest recession in decades to swing the electorate towards a candidate who pledges to stick to current economic policies.
Still, he said that the real could weaken to a record low past 4.50 to the dollar should all risks materialize - involving not only the elections, but also a failure to implement meaningful reforms this year as well as a deterioration in foreign conditions.
(Other stories from the January global foreign exchange poll:) (Reporting by Bruno Federowski; Additional reporting by Miguel Angel Gutierrez in Mexico City, Hernan Nessi in Buenos Aires, Nelson Bocanegra in Bogota, Ursula Scollo in Lima and Felipe Iturrieta in Santiago, editing by Larry King)