"Without flood insurance, residential and commercial real estate transactions in flood zones across the country have been adversely impacted, as federally backed mortgage loans cannot be secured without this critical protection, the letter from seven groups said.

"Failure to reauthorize the NFIP expeditiously when Congress returns will severely harm real estate markets, putting consumers at risk of uninsured losses and potentially putting additional tax money at risk to cover relief efforts," they warned.

Addressed to Senate Majority Leader Harry Reid, D-Nev.; House Speaker Nancy Pelosi, D-Calif.; Senate Minority Leader Mitch McConnell, R-Ky.; and House Minority Leader John Boehner, R-Ohio, the letter was sent by the American Insurance Association, National Association of Mutual Insurance Companies, Property Casualty Insurers Association of America, The Financial Services Roundtable, Independent Insurance Agents and Brokers of America, Mortgage Bankers of America, and National Association of Realtors.

"Five and a half million taxpayers depend on the NFIP as their main source of protection against flooding, the most common natural disaster in the United States," the letter said.

The organizations added, "If Congress fails to reauthorize the NFIP, it will still be paying for post-disaster relief for flood victims, yet it will be unable to collect premiums for renewing current flood insurance policies, which amounts to $2.85 billion annually. Devastating storms in the northeast underscore the need for Congressional action to reauthorize the NFIP immediately."

The NFIP was described in the letter as "critically important to Americans and the U.S. economy," and the groups urged "Congressional action now to reauthorize this program and avoid the costly consequences that would result from a failure to do so."

Republicans have based their opposition to the omnibus bill on funds not being available to pay for it.

Whether Democrats would have the votes for a cloture shutting off debate Monday to pass the bill was reportedly in doubt.

Sen. Reid's office has said that other methods would be sought to extend the programs in the legislation if cloture fails.