President Reagan, asked whether he would send combat troops to Central America, responded last night at his nationally televised news conference that "presidents never say never" about such questions.

The president quickly added that he does not plan to send U.S. troops to Central America and that the nations in the region do not want them. But in answering a follow-up question, he refused to close the door on the possibility.

" . . . It's an old saying that presidents should never say never," Reagan repeated in answer to the follow-up question. "They blew up the Maine."

Reagan's reference was to an explosion that destroyed the U.S. battleship in Havana harbor on Feb. 15, 1898, and provoked the Spanish-American War.

The president opened his 18th news conference by revising the administration's economic growth rate sharply upward from 4.7 percent to 5.5 percent annually. Details on Page A8

But the questions turned quickly to the issues that dominated the session: the Carter campaign debate book and Central America.

On another foreign policy issue, Reagan said he would consider lifting some of the economic sanctions imposed against Poland in December, 1981, in retaliation for imposition of martial law and banning of the free trade union Solidarity.

The Polish question has been the subject of intense behind-the-scenes discussion within the administration. Yesterday, Edward Derwinski, a State Department counselor with special responsibilities for Poland, suggested that the administration would settle for something less than its original conditions, which included lifting martial law, a dialogue between the government and the union movement, and freeing political prisoners.

In discussing Central America, the president issued a vigorous defense of his policies in the region, coupled with a new denunciation of the Soviet, Cuban and Nicaraguan governments and criticism of Congress for its reluctance to approve his military and economic aid requests for El Salvador.

Denouncing those in Congress who want to whittle administration aid requests to "a pittance," Reagan said: "In my opinion, what they're doing is choosing between instant death and letting those countries bleed to death. And then they want to be able to blame somebody else because they passed a nickel instead of a dollar."

He also said that while the United States has 55 military trainers in El Salvador compared with Cuba's 1,500 in Nicaragua, "all everyone seems to think is a sin is our 55."

Reagan admitted to frustration about the acknowledged decline in public support for administration policy in Central America.

He said the "constant drumbeat" of opposition since he spoke to Congress on the subject in April was having a negative effect.

"Maybe we haven't done what we should have done in keeping the people informed on what's going on there," he said.

Reagan also expressed frustration at two questions about his image as a "rich man's president" and the perception that his administration is weak in enforcing the civil rights of blacks.

Both questions took the president to troublesome issues that--along with continued high unemployment--have become major political liabilities for him under the general heading of "fairness."

In his opening statement on the economy, after trumpeting the July 1 tax cut while warning that "undisciplined" spending on social programs "threatens the recovery," Reagan took the fairness question on directly. Without mentioning unemployment, he said "fairness means encouraging and rewarding every citizen who strives to excel . . . . "

Later, when asked why most Americans surveyed in recent polls say he is a "rich man's president with no idea of what the people who aren't wealthy are going through . . . ," Reagan defended himself by saying America should be a nation where people can become rich and recalling that he was a poor youth who ate "oatmeal meat."

"My feeling--and it's very deep within me--is this," said Reagan. "No, the rich don't need my help, and I'm not doing things to help the rich. I'm doing things that I think are fair to all of the people. But what I want to see above all is that this country remains a country where someone can always get rich. That's the thing we have, and that must be preserved."

The president later was asked another version of the fairness question, this time in relation to black Republicans who have complained that civil rights enforcement under Reagan lacks "substance."

Citing efforts his administration is making, Reagan responded: "That's a little like this other question here about being a rich man's president. Someone starts creating that perception and keeps on saying it loud enough, pretty soon they get some people believing it. But there is no merit in that at all . . . . "