President Reagan, struggling to preserve his embattled foreign policy agenda, faces the first of several critical tests in Congress today when the Senate casts the decisive vote on the administration's plan to sell about $250 million worth of advanced weapons to Saudi Arabia.

On the eve of the vote, White House officials and Senate opponents of the sale predicted that the issue will be resolved by one or two votes when the Senate decides whether to override Reagan's veto of a congressional resolution disapproving the sale.

Sen. Alan Cranston (D-Calif.), who has led Senate opposition to the sale, said 66 votes are lined up to override the veto, with the outcome riding on one additional vote. If all senators are present, which Cranston said he expects, 67 votes, or two-thirds, will be necessary to override the veto and block the sale.

The sale is only one of several foreign policy issues on which congressional critics of administration policy have put Reagan on the defensive. Other than ongoing debates about the budget and tax overhaul, foreign policy questions are likely to dominate the legislative agenda in coming weeks, with the president facing difficult tasks on several fronts.

Later this month, the House is scheduled to vote again on the administration plan to provide $100 million in military and nonlethal aid to the Nicaraguan counterrevolutionaries, known as contras.

Even if an aid package clears the House this month, it is likely to include several restrictions opposed by the White House and face additional Senate opposition, including a possible filibuster.

That, in turn, would further delay an aid package that the administration said in March was needed immediately if the contras were to survive.

Meanwhile, revolts are brewing in the Senate and House about Reagan's announcement last week that the United States will end compliance with the unratified SALT II arms control agreement later this month.

Several senators, including Republicans John H. Chafee (R.I.), John Heinz (Pa.), Charles McC. Mathias Jr. (Md.) and Arlen Specter (Pa.), said yesterday that the administration should continue adherence, while House Democratic leaders agreed to a two-step strategy to counter Reagan's decision.

Rep. Norman D. Dicks (D-Wash.) said the House leaders agreed to seek adoption in the next few weeks of a nonbinding resolution opposing the decision. He described this as a prelude to attempting to amend the Defense Department authorization bill to prohibit spending for weapons deployment that would exceed SALT II ceilings.

"We want to have bipartisan support and will reach out to Republicans on this," Dicks said.

House Armed Services Committee Chairman Les Aspin (D-Wis.) said he favors initial adoption of a nonbinding resolution as an "educational device" on arms control issues, about which protracted congressional debate is likely this summer.

A fourth potential long-range problem for the administration concerns U.S. policy toward South Africa.

The White House, a Senate GOP aide said yesterday, "is definitely getting behind the power curve again" on the issue of apartheid, as it did last year when congressional agitation forced Reagan to impose relatively mild economic sanctions against the South African government rather than suffer an embarrassing defeat on Capitol Hill.

A vehicle that could provoke another showdown over South Africa exists in legislation calling for new economic sanctions, including a total ban on new U.S. investments or loans to public agencies and private concerns in South Africa.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said yesterday that he expects that the House version of sanctions legislation, sponsored by Budget Committee Chairman William H. Gray III (D-Pa.), will "sail right through" the House later this summer.

A White House official acknowledged that "we're certainly playing damage control on South Africa and any initiatives on the SALT decision." He also said that winning congressional approval of the long-delayed contra aid package will "not be easy."

Reagan continued yesterday to lobby for the Saudi arms package, telephoning wavering senators from Air Force One as he flew to South Carolina. Even if he prevails today in this first foreign policy test in the coming cycle, the victory will be only partial.

Congressional opposition to the sale forced the administration to reduce the value of the package from more than $1 billion to the $354 million plan initially proposed.

When the Senate voted, 73 to 22, and the House, 356 to 62, to reject the sale, the administration was forced to remove the package's most controversial element, shoulder-fired Stinger antiaircraft missiles that critics said could fall into the hands of terrorists.

The House vote was so lopsided that the administration has focused all of its efforts on the Senate to sustain the veto. Despite the heavy lobbying, Cranston said, "the overwhelming majority of the Senate will vote against the Saudi arms sale."

A GOP aide said, "Having only 34 senators vote for being nice to a moderate Arab country is not a great victory."