MOST READ: MasterCard, Visa Test Limits

The downturn in the economy and evidence of slowing consumer spending could put MasterCard (MA Quote) and Visa (V Quote) in the hot seat.

The financial sector has dropped to lows not seen in two decades. But the payment processing firms, each fairly new to the public markets when compared with banks and brokerages, have been labeled as two bright spots, since they have largely avoided pain from the credit crisis and housing fallout.

However, as the economy deteriorates, some observers are getting nervous. They question whether the two companies' strategy of capitalizing on consumers' increasing reliance on plastic to pay for purchases will be able to withstand the consumer troubles in the U.S. and potentially abroad.

Investors in MasterCard and Visa will be listening to hear how the firms characterize the depth of the downturn in the U.S. and will be on the watch for any forward-looking comments on their businesses, observers say.

"This is the quarter when the resiliency of MasterCard's business model will be tested, as the various data points we monitor all tracked somewhat weaker/slower in the quarter," writes Howard Shapiro, an analyst at Fox-Pitt Kelton Cochran Caronia Waller, in a research note where he trimmed his quarterly earnings estimates on the firm. "These include weak retail sales in the U.S. and slower growth in revolving debt, anecdotal signs of slowing volume growth internationally and a flat dollar versus major currencies."

Adil Moussa, a payments industry analyst at Aite Group, says he is more worried about Visa than about MasterCard because the San Francisco-based firm is more concentrated -- Visa Europe is a separate entity. This leaves Visa "more at risk" and "at the mercy of economic downturn" than MasterCard, he says.