“The government has succeeded in driving mortgage rates down to their lowest level in our lifetime,” said Guy Cecala, the publisher of Inside Mortgage Finance magazine. “That hasn’t been a big home run, because a lot of people can’t take advantage of it.”

It is highly unusual for mortgage money to be available below 5 percent. Average rates fell as low as 4.7 percent in the 1940s, as the government held down interest rates to finance World War II, and stayed just below 5 percent until the early 1950s. Rates went above 5 percent in 1952 and stayed there  until this year.

The super-low rates are not likely to last much longer. The Federal Reserve program that has driven rates to such lows, which involves buying $1.25 trillion in mortgage-backed securities, is scheduled to expire in March, and Fed leaders have said that it would not be renewed.

Some analysts believe rates could jump as high as 6 percent in the spring. On a $300,000 mortgage, such a jump would cost an extra $225 a month.

Andrew Knapp, a sales executive in Bartlett, Ill., has tried twice to refinance, which would save his family several hundred sorely needed dollars every month. Lenders said the house had lost value and the Knapps had too much debt. “There was no urgency for them to do anything,” Mr. Knapp said.

The most recent Federal Reserve survey of lenders found that they were continuing to tighten terms for business and household loans. Banks say they are under pressure from regulators to raise their cash reserves, which means fewer loans. They also argue that a troubled economy breeds extreme caution.

“More than ever before, lenders are very conscious of making good quality loans,” said Michael Fratantoni, the vice president for research at the Mortgage Bankers Association. “They are looking at the value of the collateral and the credit quality of the borrower.”