By Phil Stewart

BRASILIA, July 26 (Reuters) - U.S. President Joe Biden's defense secretary, Lloyd Austin, is expected to call on militaries to respect democracy at an Americas-wide defense gathering this week in Brazil, a senior U.S. defense official said.

Those expected remarks - while not specifically directed at Brazil - are likely to turn heads there ahead of its Oct. 2 election, where Brazilian President Jair Bolsonaro formally launched his re-election bid on Sunday by saying "the Army is on our side."

The right-wing populist has publicly cast doubt on the validity of Brazil's electoral system, and some polls show him down almost 20 percentage points against former President Luiz Inacio Lula da Silva of the Workers Party.

Austin, a retired Army general, will address the bi-annual defense ministers' conference and hold bilateral talks with regional delegations on Tuesday and Wednesday.

"To the venue as a whole, he is going to bring a very strong and clear message on the need for militaries to respect democracies," a senior U.S. defense official said, speaking on condition of anonymity. The official declined to "prejudge" what Austin might say, however, to his Brazilian counterparts.

Bolsonaro, a former Army captain, told diplomats earlier this month that the Brazilian military should be called in to help secure transparency in the Oct. 2 election. He has pushed electoral authorities to accept a parallel vote count to be carried out by the armed forces. They have ruled that out.

The maneuvers have unnerved Brazil-watchers in Washington, including in Congress.

"(Austin) should simply make clear that the military should stay out of the election and allow any disputes about the election to be resolved by constitutional means," U.S. Congressman Tom Malinowski, a Democrat and member of the House Foreign Affairs Committee, told Reuters.

"And he should remind his counterparts that U.S. law restricts our cooperation with foreign militaries that participate in anything that might resemble a coup."

Military leaders have repeatedly said Brazil's armed forces will respect any result of the election.

Some military officials have made headlines, however, by echoing Bolsonaro's comments about potential weaknesses in Brazil's voting system.

"POLITICAL TEMPEST"

Bolsonaro has based much of his political career on nostalgia for Brazil's 1964-1985 military dictatorship, disparaging Congress and the courts while filling his government with current and former officers from the armed forces.