WASHINGTON — The cascading fallout from the subprime loan crisis, barely a cloud on the horizon a year ago, is now viewed by experts as the economy’s gravest threat.

In a survey being released today, 34% of the members of the National Association for Business Economics ranked the financial market turmoil from those loan defaults as the no. 1 threat to the economy over the next two years. That compares with 18% from an August survey, when the most serious threat was seen by 20% of the economists as terrorism and the conflicts in the Middle East. A year ago, the credit crisis did not even register as a chief threat. The latest survey found that 18% of association members listed excessive debt held by households and businesses as the top problem.

The questioning of 259 economists took place during the first two weeks of February. Events since then have underscored the credit crisis problems.

On Friday, the Dow Jones industrial average plunged by 315.79 points. The decline resulted from a combination of grim economic news, including a new estimate from UBS Securities analysts that the financial system losses from securities backed by mortgages and other debt would total $600 billion. That far surpassed the $400 billion that many economists projected until recently.

At the heart of financial institutions’ problems are securities backed by subprime mortgages. They have gone into default at record rates because of the housing market’s steep slump. These loans were extended to borrowers with weak credit histories.

A separate 49-member NABE forecasting panel recently raised its expectations of a recession, with close to half thinking a downturn will start before year’s end. But 55% of the forecasting panel still thinks a downturn can be avoided with the help of an $168 billion economic aid plan and aggressive interest rate cuts by the Federal Reserve.