President Reagan said last night that the United States "cannot turn away" from the rebels opposing the leftist government of Nicaragua and added that if they are given financial assistance, they can fight the Sandinistas effectively without the support of U.S. troops.

"They are our brothers, these freedom fighters, and we owe them our help," Reagan said in an emotionally charged speech to the 12th annual Conservative Political Action Conference. ". . . They are the moral equivalent of the Founding Fathers and the brave men and women of the French Resistance. We cannot turn away from them. For the struggle here is not right versus left but right versus wrong."

In the past two weeks Reagan has made a radio speech appealing to Congress to release $14 million in aid to the rebels, known as "contras," and said at a news conference that he favors removal of the Managua government unless it includes members of the rebel groups and restores basic freedoms.

Last night, while reiterating these aims, Reagan also spelled out his opposition to using U.S. troops to overthrow the Sandinistas.

"I am against sending troops to Central America," the president said. "They are simply not needed. Given a chance and the resources, the people of the area can fight their own fight . . . . All they need is our support. All they need is proof that we care as much about the fight for freedom 700 miles from our shores as the Soviets care about the fight against freedom 5,000 miles from theirs." Reagan's speech, to a sympathetic audience, celebrated the values and political victories of what he called "the conservative movement." He said that "perhaps the greatest triumph of modern conservatism has been to stop allowing the left to put the average American on the defensive."

The president restated his basic opposition to communism and his commitment to "freedom movements" in the world, and defended his proposal for a space defense system known as "Star Wars."

But White House officials said that his speech was toned down considerably from the original draft submitted by communications director Patrick J. Buchanan, who now oversees the president's speechwriting team. Earlier in the day, White House chief of staff Donald T. Regan brought approving laughter when he said that Buchanan gave "new meaning to the word 'hard-core.' "

Sources said the original draft of the speech contained several harsh anticommunist references reminiscent of Reagan's first-term descriptions of the Soviets as an "evil empire" and the "focus of evil."

One White House official said that the references, one of which referred to communists as "thugs," were "unpresidential" and were deleted. Another said that some of the phrases in the first draft reflected "excessive exuberance."

Two sources said that national security officials objected to a speech that escalated anti-Soviet rhetoric during the same month that the United States and the Soviets are resuming arms-control negotiations in Geneva.

What survived was a speech that traced the ascendancy of conservative values, which Reagan said had prevailed "because the other side is virtually bankrupt of ideas; it has nothing more to say, nothing to add to the debate."

"We in this room are simply profiting from their bankruptcy," Reagan told his audience at the Sheraton Washington. "We are where we are because we are winning the contest of ideas."

Reagan portrayed the conservative majority as composed of "average Americans," whom he defined as "the good, decent, rambunctious and creative people who raise their families, go to church and help out when the local library holds a fund-raiser . . . . These people had held true to certain beliefs and principles that for 20 years the intellgentsia were telling us were hopelessly out of date, utterly trite and reactionary."

Reagan said conservatives have "captured the moment, captured the imagination of the American people," and went on to proclaim this a prelude to a "golden age of freedom."

While the president clearly was the hero of the convention, and many of his major policies were seen as beyond question, there were signs yesterday of lessening conservative enthusiasm for the arms buildup that has been a feature of his presidency.

At a morning panel, Sen. Charles E. Grassley (R-Iowa) issued a spirited challenge to Reagan's arms buildup, charging that flaws in the Pentagon's procurement procedures "present a clear and present danger to our ability to met our defense needs and obligation."

Grassley, a conservative elected to the Senate in 1980 after three terms in the House, said he originally voted for Reagan's defense programs but now believes the defense budget should be frozen to force changes in Pentagon purchasing practices.

"The weakness of conservatives too often has been to equate a larger defense budget with a stronger defense," he said. "The evidence is clear -- we're spending more and getting less in some cases than we did in the Carter years."

Grassley said that in Reagan's first term, spending on Navy and Air Force aircraft had increased 75 percent in constant dollars over what was spent in the Carter administration, but that 11 fewer planes were produced. He said the Pentagon now has 30 percent more tanks and 40 more helicopters than it did four years ago, but that it required an 85 percent increase in spending to accomplish it.

"We are paying for 17 Maverick missiles for every one we see," he said, blaming this on inefficiency and a lack of competition for defense contracts.

"The very word 'competition' is alien to the tight, closed world of defense spending," he added. "I am for a stronger defense. I simply want those defense dollars to do what they are supposed to."

The conference continues today. Salvadoran rightist leader Roberto D'Aubuisson was invited to address the convention tonight, but the State Department refused to allow him to enter the country. He will be replaced by Eden Pastora, a former Sandinista military officer who now heads a group of rebels opposing the Nicaraguan regime.