NEW YORK (Reuters) - Oil prices slipped for a third trading session on Tuesday as investors focused on the prospect of swelling oil supplies and weak demand, while showing little reaction to U.S. presidential campaign upheaval.

Brent crude futures for September fell 9 cents to $82.31 a barrel by 0005 GMT. U.S. West Texas Intermediate crude for September declined 10 cents to $78.30 per barrel.

Traders mostly ignored U.S. President Joe Biden's decision to call off his reelection bid and endorse Vice President Kamala Harris on Sunday. Citi analysts said they believed neither Harris nor Republican nominee Donald Trump would promote policies that would greatly affect oil and gas operations.

Instead, the market focused on oil supply and demand, which Morgan Stanley analysts said was likely to balance out by the fourth quarter and rise to a surplus by next year, which would drag down Brent prices to the mid-to-high $70s per barrel range.

The American Petroleum Institute, a trade group, is due to release its estimates for last week's oil inventories on Tuesday, while official U.S. government data is scheduled to land on Wednesday.

A preliminary Reuters poll of six analysts estimated that U.S. crude stocks, on average, fell by 2.5 million barrels in the week to July 19, while gasoline stocks likely dropped by 500,000 barrels.