Lingling WeiMarket enthusiasm for the Federal Reserve's plan to help the commercial-real-estate market was doused Tuesday after Standard & Poor's warned that billions of dollars of top-rated bonds backed by commercial mortgages could face downgrades.

The financing drought in commercial real estate is threatening to cause major damage to the economy, so the sector was cheered when the Fed announced in March that it would expand one of its main rescue programs to help resuscitate the $700 billion market for bonds backed by mortgages...