President Reagan and Secretary of State Alexander M. Haig Jr. appealed for bipartisan support of foreign aid yesterday as Congress approached a moment of truth in which Republican votes will be crucial.

The president endorsed two pending foreign aid bills in letters to House leaders and Haig made an unusual personal appearance in the Capitol to urge traditionally reluctant Republican members to back their president's request.

Their efforts marked the first time in the current budget confrontation that the administration, which has urged sharp cuts in domestic spending, has also come out so publicly at the highest levels in behalf of foreign aid.

In a separate development, the administration lost one round yesterday in its effort to gain flexibility for foreign aid to Pakistan. The House Foreign Affairs Committee, ignoring a State Department plea, voted to give Congress a stronger veto power over aid to countries believed to be developing nuclear weapons. Aid to Pakistan was banned under the Carter administration because that country is suspected of developing such weapons.

The foreign aid authorization bill and a $11.1 billion appropriation come before the House this week with more than 100 Republican votes needed to pass them. Republicans have been hostile to foreign aid in recent years, leaving its passage largely to Democrats who are often pilloried for voting overseas "giveaways."

Vote-counters yesterday gave varied predictions. After Haig's appeal to Republicans on the House floor, White House lobbyist Max L. Friedersdorf said, "It's hard to tell if we have the votes. There are lots of undecideds. But I think his Haig's arguments were persuasive."

However, Rep. Edward J. Derwinski (R-Ill.) predicted that 130 or 140 Republican members would "bite the bullet" and vote for foreign aid.

"They'll do it in the context of national defense," Derwinski said. "That's the glue holding it all together."

That was a reference to the administration's strategy of picturing foreign aid as a key element in militarily shoring up allies, particularly in the Middle East.

Reagan talked up foreign aid in a White House meeting Monday night with 14 GOP members of Congress and plans a similar session this morning with the House Republican leadership, Friedersdorf told reporters. "The president is emphasizing the security part of foreign aid and is putting less emphasis on the economic part of it," he said.

In his letter to House leaders yesterday, Reagan said the current legislation "represents a carefully balanced approach to economic and security assistance."

The administration has embraced both the authorization and appropriation bills developed by House Democrats. It would like to see the military component increased substantially and opposes some parts that restrict presidential powers, but has decided to go with the Democratic-sponsored measures as the only ones available in the House.

The Republicans are expected to attempt to trim spending on multilateral economic aid, particularly for the International Development Association. The House bill includes $850 million for that program. The Senate has approved $520 million and House Republicans will try to cut it back to that lower level.

Republicans may also attempt to recoup from the loss suffered in the Foreign Affairs Committee, which made it easier for Congress to overrule the president on aid to Pakistan or other countries that try to develop nuclear weapons.

Current law named for former senator Stuart Symington (D-Mo.) prohibits foreign aid to countries that are known to be developing nuclear weapons. It has been invoked so far only against Pakistan, for which the administration is planning a $3.2 billion aid program, including $100 million in this fiscal year.

The law now permits a president to waive the restriction if he believes it is in the national interest to do so, though Congress can also override him.

An amendment by Rep. Stephen J. Solarz (D-N.Y.), which the committee adopted unanimously, would permit Congress to override the waiver by a concurrent resolution, requiring only simple majorities and not subject to presidential veto.