A senior White House official acknowledged yesterday that President Reagan's proposed defense budget, the subject of a nationally televised address Wednesday night, is in "deep trouble."

The official, who was interviewed on grounds he not be identified, was echoing what bipartisan congressional leaders have been saying in recent months about Reagan's proposal for an 8 percent increase in defense spending above inflation. "We're in deep trouble, as you know," he said, "meaning that three of four people think we already have enough defense."

Other officials said White House chief of staff Donald T. Regan has recently been taking soundings on Capitol Hill about the prospects for the president's budget as deadlines approach next month for congressional action. "We're a long way from victory," one official said. Another said, however, that Reagan is not ready to "bargain" with Congress over defense cuts.

The president reiterated yesterday he is not ready to compromise on defense. "The way I see it, this is no time to back down, backtrack or backslide," he told a group of journalists.

But Reagan also acknowledged that the public consensus for defense spending has slipped, partly as a result of his military buildup. "We haven't caught up" with the Soviets, he said. "I suppose what's on my mind is the fact that a recent poll and a very authoritative poll showed that the overwhelming majority of the American people believe that we have now caught up, passed them and are superior in military force to the Soviet Union. And that is just not true . . . ."

Defense Secretary Caspar W. Weinberger told the same group that Reagan's request "certainly isn't excessive based on what the Soviets have and what they will spend."

Also yesterday, Reagan sought to link the "triumph of the people's will" in the Philippines with his proposal for $100 million in military and nonlethal aid to the rebels fighting the Sandinista government of Nicaragua. He said the Sandinistas, Soviets and Cubans "have decided to make a big push now and finish off" the rebels. But he also said the "freedom fighters have never been stronger. They have more men in the field and more support than ever from the people."

Reagan said the ideal outcome in Nicaragua would be a negotiated settlement with the rebels, known as contras or counterrevolutionaries. But he also said, in response to a question, that the contras could eventually win a military victory.

"If the Sandinista government is going to fight to the death and to the last man, I suppose that would be the outcome," he said, adding that a military victory "is not the primary goal" of the rebels.

Questioned again, Reagan said he thought a military victory by the 20,000 rebels was possible because "the Nicaraguan government forces, regardless of their great strength in armaments, they are losing the support of the people. A great many of those . . . contras now are deserters from the Sandinista armed forces."

At another point, Reagan was asked about the communist insurgency in the Philippines. "You can't just say ho-hum, there aren't very many of them," he responded. "They're a little like rabbits. So . . . you've got to take them seriously. And you can't have a coalition with them."