President Reagan, having walked through sun-blistered fields with Illinois farmers, says he is itching to sign drought relief legislation - and Congress has taken the first step to send it to him.

One day after being confronted with dozens of amendments threatening to slow the process, the House and Senate Agriculture committees pushed them through Thursday and wound up with bills very similar to the "core" legislation filed early this week. The Senate panel sent its bill to the floor for debate, and the House panel gave tentative approval and agreed to meet again July 26.Reagan made a flying tour of Illinois and Iowa Thursday and called at each stop for speedy action to help drought-stricken farmers, who face "the worst natural disaster since the Dust Bowl of the 1930s."

"(Lawmakers) know there's a time pressure on it," the president said in a broadcast interview in Davenport, Iowa. "I think it's ready to go through and, of course, I'll sign it the minute it's delivered to me."

The trip marked the first time Reagan publicly specified what he wants in drought legislation. He already had given approval to the $5.5 billion effort introduced in Congress, which was worked out in bipartisan discussions between congressional leaders and the administration.

Aides said the president's comments, including his criticism of any lawmaker trying to attach special-interest language to the bill, were intended to keep Congress from bogging down in amendments or bloating the price of the bill.

House Speaker Jim Wright, D-Texas, has put the drought measure on the House calendar for action when Congress returns from its weeklong recess for the Democratic National Convention.

Agriculture Secretary Richard Lyng, who accompanied Reagan Thursday, said the bill sent to the Senate floor appeared "fairly close" to the original package.

The House committee made a few changes, including a requirement for farmers who get disaster aid this year to buy crop insurance the next two years. The committee also voted to make all stricken farmers eligible for low-interest Farmers Home Administration loans. Currently loans go only to those with crop insurance.

In addition, the Agriculture Department announced all 114 counties in Missouri had been declared natural disaster areas because of the drought, making the FmHA emegency loans available.

Reagan joined Herman Krone and his son, Rick, in examining drought-shrunken corn and soybean fields at the DuQuoin State Fairgrounds in Southern Illinois. A pole showed the corn should be well above the president's head, but it was only chest-high. Reagan plucked a tassle from the top of a corn stalk and walked with the Krones to a soybean field where the plants were shin-high. In a normal year, they would be between 3 feet and 4 feet tall.

"What I saw was not a pretty sight," Reagan then told 85 farmers and local officials invited to a briefing under two cottonwood trees. "Stunted corn, sparse bean fields, withered plants starved for water, struggling to push their way up."

Reagan said disaster relief must allow drought-hit farmers to keep the crop subsidy payments they already have received this year and must open up aid to those such as fruit and vegetable growers who normally do not qualify for any federal farm benefits.