WASHINGTON (AP) — The United States could default on itsobligations as early as Oct. 18 if Washington fails to agree onlegislation to raise the government's borrowing cap, a new studypredicted Tuesday.

The Bipartisan Policy Center analysis says the default datewould come no later than Nov. 5 and that the government wouldquickly fall behind on its payments, including Social Securitybenefits and military pensions.

The think tank's estimate is in line with a warning last monthby Treasury Secretary Jacob Lew that the government would exhaustits borrowing authority by mid-October and be left with just $50billion cash on hand.

The government has never defaulted on its obligations. Raisingthe $16.7 trillion borrowing cap promises to be a major strugglefor House Republicans and President Barack Obama.

Two years ago Obama agreed to pair a $2.1 trillion increase inthe debt limit with an equivalent amount in spending cuts spreadover 10 years. But the president now says that he won't negotiateover the debt limit and is asking Congress to send him astraightforward increase that would ensure the government can payits bills.

In January, House Republicans permitted an increase in the debtceiling without demanding offsetting spending cuts.

It's commonly agreed that failure to increase the debt limit ontime would roil financial markets and lead to a downgrade of thegovernment's credit rating. The political fallout would also beintense, especially if Social Security benefits are delayed.

Tuesday's study predicts that if the default date — which iswhen the government cannot pay its bills in full and on time —comes on Oct. 18, the subsequent Social Security payments due onNov. 1 could be delayed by almost two weeks.