That's the highest proportion of the lower 48 states classified in drought since the inception of the monitor in 2000, the National Weather Service said Thursday.
Dry conditions and drought, worsened by above-normal temperatures, have been expanding and growing more intense in much of the central and northern United States.
The combination of heat and dryness has taken a major toll on the quality and quantity of the corn and soybean crops. The United States Department of Agriculture said 38 percent of corn and 30 percent of soybeans were rated poor or very poor as of Sunday.
In states including Kentucky, Missouri and Indiana, more than 70 percent of corn was adversely rated.
A worsening drought ravaging much of the nation is spoiling crops and will likely hit Americans at the supermarket, U.S. Agriculture Secretary Tom Vilsack said.
"I get on my knees every day and I'm saying an extra prayer now," Vilsack told reporters at the White House after discussing the crisis with President Barack Obama.
"If I had a rain prayer or rain dance I could do, I would do it," he said, calling the crisis "the most serious situation" in about 25 years.
Vilsack said 1,336 counties -- more than a third of all 3,033 U.S. counties or county-equivalent governments -- were designated disaster areas.
He said more than 75 percent of U.S. corn and soybean crops were in drought-affected areas.
The nation may still have a bumper corn crop this year -- the third-largest corn crop in history -- because favorable weather early in the growing season encouraged planting, The New York Times said.
But Vilsack said the drought would result in food price increases into 2013.
Milk and cheese prices will probably rise first, followed by corn and meat, industry analysts told USA Today.
The price of corn has increased 38 percent in recent weeks and the price of beans is up 24 percent.
The administration has lowered the interest rate for emergency loans to farmers and is streamlining farm-assistance programs, Vilsack said.
Congress could help by revising disaster aid programs that expired last year, Vilsack said. It could also help by providing assistance through the Local Farms, Food and Jobs Act, a bill introduced in November and intended for inclusion in the 2012 Farm Bill.
The Farm Bill was introduced in the Democratic-controlled Senate May 24 and passed that chamber June 21. It is currently being considered in the Republican-controlled House.
Meanwhile, the Weather Service said recent widespread thunderstorms in the southeast have eased drought conditions in that region.
The three-month outlook calls for above-normal precipitation along the central Gulf of Mexico. As a result, some improvement in drought conditions is expected in the Deep South, from coastal Texas eastward to South Carolina.
In the southwest, conditions were expected to improve in Arizona and western New Mexico as the summer monsoon continues. But drought is likely to persist in the rest of the western United States, as late summer and early fall are normally dry.