If the Senate votes today to disapprove President Reagan's proposed $8.5 billion aircraft sale to Saudi Arabia, many congressional sources believe that existing law would permit him to circumvent Congress' opposition and carry out the sale.

Congress has the power to veto any foreign arms sale of $25 million or more if both houses adopt an identical resolution of disapproval within 30 days after the executive branch formally notifies Congress of its intention to make the sale. In respect to the Saudi deal, the House already has adopted such a resolution.

However, congressional sources say a way around such a veto apparently is provided in a waiver provision added by Congress last December to the Foreign Assistance Act. It allows the president to make sales when he "determines and so notifies in writing the Speaker of the House of Representatives and the chairman of the Committee on Foreign Relations of the Senate that to do so is vital to the national security interests of the United States."

In addition, the Arms Export Control Act contains a provision allowing the president to go ahead with an arms sale if he states that "an emergency exists" affecting national security. He must give Congress "a detailed certification for his determination, including a description of the emergency circumstances...and discussion of the national security interests involved."

That provision always has been interpreted as applying to situations where a U.S. ally is facing a crisis such as an external attack or internal rebellion. Because the Saudi government is not faced with such an emergency, congressional sources believe that the president would not be able to justify the Saudi sale under the Arms Export Control Act.

But, the sources add, last year's amendment to the Foreign Assistance Act is a different matter, and they concede that the only apparent bar to its use by the president would involve the political question of whether he wishes to incur the ill will of majorities in both houses of Congress.

Reagan has not specifically ruled out resorting to this loophole, but he has said he is not contemplating its use while the matter is before the Senate. That position was reiterated Monday by deputy White House press secretary Larry Speakes.