Heading into the year 2013, the U.S. financial system is about as sound as it's ever been, but the threat of the fiscal cliff could derail gains made within the banking system, said Mark Zandi, chief economist for Moody's Analytics.

Zandi says tighter underwriting on mortgages, as well as other consumer loans, has improved credit conditions. In addition,  Americans have paid down their debts while obtaining the monthly cost-saving benefits of refinancing their mortgages.

Depository institutions also have billions in new capital and benchmark ratios are at all-time highs, Zandi said.

"The quality of commercial and industrial loans, credit cards, and auto loans is about as good as it gets," he wrote. "Even among first mortgage loans, the number delinquent between one and two months are at record lows. The missing ingredient for stronger bank profits is more lending, and banks are steadily opening the credit spigot."

So what could derail these gains in 2013?

Zandi cites U.S. lawmakers failure to reach an extension on the debt ceiling and an agreement on tax cuts, which are set to expire in January, as a scenario that could create enough fiscal drag in early 2013. Either way, the first part of the year is cased in some uncertainty.

"The impediment to growth will be significant – particularly during the first half of next year – but manageable," Zandi said. "With some additional clarity from policymakers on taxes and budgets, businesses, banks and households should all become more aggressive in investing, hiring, lending and spending."