Exxon and Microsoft bonds: The real safe haven

NEW YORK (CNNMoney) -- The fight over raising the federal government's debt ceiling could end up making it more expensive for businesses to borrow money.

Credit rating agencies Standard & Poor's and Moody's have warned that they might downgrade the U.S. if the debt ceiling is not raised by the August 2 deadline.

But experts say that even if the United States loses its AAA rating, that won't lead to immediate downgrades of U.S. companies.

The four companies that currently have AAA ratings -- Exxon Mobil (XOM, Fortune 500), Microsoft (MSFT, Fortune 500), Johnson & Johnson (JNJ, Fortune 500), and Automatic Data Processing (ADP, Fortune 500), would be able to preserve a better credit rating than their home nation. That happens frequently elsewhere in the world.

John Bilardello, head of corporate ratings for Standard & Poor's, said his firm will have to selectively reevaluate many corporate credit ratings to see which companies would be hurt by deep cuts in government spending.

Bilardello said if Congress remains deadlocked after the deadline to raise the debt ceiling, that would force the government to immediately halt making many payments. That could cause an economic shock that would force a reassessment of credit ratings across a wide range of industries.

But even if an agreement to raise the debt ceiling is reached in the next few days, there's still a chance that the nation's credit rating could be cut. S&P is on record saying it could downgrade the U.S. without a deal to make significant reductions in long-term government debt.

That would be bad news because any downgrade could raise the interest demanded by investors on U.S. Treasuries. And any such increase would likely bleed through to rates paid on most corporate bonds as well.

Kevin Giddis, managing director of fixed income at Morgan Keegan, said if no deal is reached by August 2, the spreads between lower-rated corporate debt and Treasury bonds could widen substantially.

Bilardello agreed that failure to raise the debt ceiling could hurt the companies with the weakest credit ratings the most.

But the handful of firms with pristine credit ratings may, as strange as this sounds, actually benefit. Giddis noted that there are rules for some asset managers which require that they hold only AAA-rated debt.

Those investors could be forced to sell their Treasuries and move into AAA-rated corporate debt. He said it's possible that some high-quality corporate bonds could even end up with yields below that of Treasuries.

"There's a scenario you could have where Exxon Mobil debt would be worth more than U.S. debt," he said.

But Giddis said he expects the vast majority of institutional investors will continue to hold the Treasuries now in their portfolios, even if there is a downgrade.

He speculated that if fund managers were to make any changes in their holdings, it would be to sell off other lower-rated bonds in order to keep their average debt rating unchanged.