If Congress refuses again to fund the Reagan administration's program of aid to antigovernment rebels in Nicaragua, nearly 150,000 refugees will flee that country and swamp Costa Rica and Honduras, a senior administration official said yesterday.

The warning, the first time such a possibility has been mentioned, came as another official, in California with President Reagan, announced that Reagan will make a strong pitch for the program in his weekly radio address today.

A senior State Department official, briefing reporters here on condition that he not be named, said the White House is optimistic that Congress will provide funds. "I think there's been a shift of opinion up there," the official said. "A lot of strong emotions . . . are involved, and people don't want to change their votes. But they might be amenable to finding some other way."

The combined statements represented an apparent opening salvo in the administration's drive to win congressional approval of $14 million in aid for the rebels, a battle the officials said is expected to begin in earnest when Congress finishes dealing with the MX missile. Congress halted funds for the three-year rebel-aid program last May.

In an interview, the first official said about 15,000 rebels fighting Nicaragua's leftist Sandinista government "would be many, many more if they had arms to give them." If Congress refuses further help for the rebels, he said, "whole villages in Nicaragua would pick up and move across the border" into Costa Rica or Honduras.

Such a development, he said, would be "the collapse of a whole movement. A lot of people would feel very scared, very exposed. It would be a massive refugee problem" involving perhaps 150,000 people, he said, and the administration "would have to go back to Congress to deal with it."

The argument has echoes of the "feet-people" scenario two years ago when administration officials warned that millions of refugees would flee toward the U.S. border from communist advances in Central America. It also reflects the administration's position that, to preserve its international credibility, the United States must publicly maintain commitments it may have made privately.

Speaking in California, the second administration official said Reagan would discuss the rebels, called "contras," in his radio broadcast and "the necessity for our giving financial aid -- and I underscore the word 'financial' -- to these contras" to press the Nicaraguan government for reform.

Critics of the aid program have argued that it is illegal to provide arms to rebels against a nation with which the United States is formally at peace. The stress on "financial" aid could be an administration response to that objection, a possibility the State Department official seemed to confirm in listing nonlethal aid as an option.

The second official added that the president would compare aid to the rebels in Nicaragua with aid U.S. rebels against Britain received from the French in the 1770s.

"Why shouldn't we support them? Do you think we should allow another Cuba in . . . Central America, which is where we're heading if we don't?" he asked rhetorically.

On another matter, the State Department official said the United States will have opening positions on all three aspects of arms control -- intermediate and long-range missiles and space weapons -- when talks begin in Geneva next month. He added that interagency preparatory work "is fairly far along." But, he cautioned, "we don't expect final decisions on our positions until very shortly before our negotiators leave for Geneva."