WASHINGTON (AP) _ The Reagan administration's bid to sell Kuwait 40 advanced F-18 fighters is running into strong congressional opposition as the Senate votes to strip a potent missile from the warplane's weapons inventory.

The Senate action to remove the air-to-ground Maverick missile from the $1.9 billion arms package came only hours after the administration formally asked Congress to approve the sale to give oil-rich Kuwait a ''first line of defense'' in a region racked by the war between Iran and Iraq.

Before the measure was adopted on a voice vote, senators made clear they believe the Maverick would pose a security threat to Israel if it should fall into the hands of the Arab enemies of the Jewish state.

Earlier Thursday, before a House Foreign Affairs subcommittee, administration officials said the Maverick was an essential part of the F- 18's defensive systems and part of an arms package so structured as to pose no threat to Israel.

But critics said the sale would fuel an escalating arms race in the volatile Persian Gulf.

Sen. Dennis DeConcini, D-Ariz., at first attempted to bar the sale of Mavericks to any nation in the Persian Gulf region, then modified his proposal to apply to Kuwait only.

Sen. Arlen Specter, R-Pa., said that if he had his way he would condition the sale of the Maverick and other ''state-of-the-art'' weapons to Middle East nations only if they ''renounce the use of force and recognize the right of Israel to exist.''

Congress has 30 days to block the F-18 sale. Otherwise it goes ahead.

Although there is wide opposition in Congress to selling sensitive high technology weapons to Middle East nations it was not clear whether a ban on transferring Mavericks to Kuwait will eventually become law.

Testifying before a House Foreign Affairs subcommittee, Peter Burleigh, assistant deputy secretary of state for Near Eastern affairs, described the Maverick as an essential element in a purely defensive package.

Burleigh said the deal is in the interests of the United States which is using naval forces to protect the ships of Kuwait and other nations sailing through the Persian Gulf.

''Friendly states should be their own first line of defense, so we don't have to be,'' Burleigh said.

''By helping a moderate gulf state assume greater responsibility for its own defense, it complements our highly successful escort operations,'' he said.

Burleigh also testified that a group of U.S. allies will soon announce a major arms sale to Saudi Arabia, another major oil producing Arab state on the Persian Gulf.

One subcommittee member, Rep. Larry Smith, D-Fla., described the pending sale to Saudi Arabia as ''enormous,'' running into the ''tens of billions of dollars.''

Burleigh said he was not able to disclose which countries are involved in the deal, the nature of the weapons to be sold or their total value.

The Reagan administration seeks to sell Kuwait not only the 40 F-18s but their armaments as well. Other than the Maverick, the weapons include Sidewinder, Sparrow and Harpoon missiles and cluster bombs.

The missiles are among the most advanced air-to-air and air-to-surface weapons in the U.S. inventory.

But Burleigh said only limited quantities of the missiles are being provided, ''far less than would be necessary for an offensive purpose.''

Under the terms of the agreement with Kuwait, the first of the aircraft would be delivered in 1991 and the remainder over the next three years.

Responding to critical questioning, Burleigh said F-18s in Kuwait will not have the range required to strike at targets in Israel and that Kuwait will not be sold the tanker aircraft needed to increase that range.