NEW YORK, July 23 (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.