WASHINGTON — Pressure is mounting from both Republicans and Democrats in Congress for President Bush to guarantee that a proposed $20 billion arms sale to Saudi Arabia will not arm Riyadh with weapons it could turn on American soldiers or Israel.

Rep. Mark Kirk, a Republican of Illinois, and Rep. Christopher Carney, a Democrat of Pennsylvania, are circulating a letter calling on the White House to “guarantee to Congress’ satisfaction that selling JDAMs to Saudi Arabia will not harm U.S. forces or our democratic ally Israel.”

Joint Direct Attack Munitions are the global positioning systems and steering technology an air force can place on unguided bombs to improve their accuracy.

The letter from Congress, set to be sent to Mr. Bush on Friday, comes during a week in which the State Department formally notified Congress of the weapons sale to Saudi Arabia. Some House members, led by Rep. Anthony Weiner, a Democrat of New York, have already announced that they will oppose the $20 billion sale.

The lawmakers who signed on to the letter from Messrs. Carney and Kirk do not go that far. They do say, however, that if they are not assured the weapons cannot be turned on America or Israel, “we will oppose the sale.”

The opposition from Congress has the potential to scuttle a key component of Mr. Bush’s new strategy to counter a rising Iran. The arms sale, along with the upcoming Arab-Israeli peace conference scheduled for later this month in Annapolis, Md., is meant to create and an American-Sunni Arab alliance against Iran, whose regime Mr. Bush has accused of killing American soldiers in Iraq, destabilizing Lebanon, and building a nuclear bomb.

Mr. Kirk said yesterday that he was concerned that Mr. Bush could be making the same mistake with Saudi Arabia that President Carter made when he sold F–14 Tomcat Fighter Jets to the shah of Iran in 1977 and 1978. After the Islamic Revolution in 1979, those fighters were aimed at the U.S. Navy.

“Over the long haul, unless you see a stable democracy, you have got to contemplate the prospect of a government changing. You want to make sure any advanced weapons you provide cannot represent a danger to Americans in uniform,” he said.

Mr. Kirk declined to detail the technical guarantees that he and his colleagues are asking of the Bush administration. Other sources close to Congress, however, said changes could be made to the JDAM guidance system and global positioning system that would make it impossible to turn the munitions toward Israel.

A former executive director of the American Israel Public Affairs Committee, Morris Amitay, said, “In the past, the administration has tried to ease Israeli concerns by modifications to the equipment sold so as not to pose a new threat to Israel.”

Such a modification was made in the 1970s, when America sold Hawk anti-aircraft missiles to Jordan that were set in concrete, rendering the missile system immobile so that it could not be moved to the Jordan River Valley.

Aipac did not endorse a letter from Mr. Weiner in August opposing the Saudi arms sale, but it offered support yesterday for the letter from Reps. Carney and Kirk. The group “supports the Kirk-Carney letter and efforts by Congress to seek assurances from the administration that ironclad measures will be included in the final arms package to ensure the safety of American troops and our allies in the region,” a press secretary for Aipac, Jennifer Cannata, said.

Israeli leaders have expressed support for the arms package, which is part of a larger deal whereby Israel is promised $30 billion in American defense assistance over the next 10 years. Last month, Israel and America signed a new memorandum of understanding reaffirming that Washington was committed to Jerusalem’s qualitative military advantage over the Arab states.