President Reagan tried to reassure conservatives last night that he will not be a one-term president, telling a group of longtime supporters that "our cleanup crew will need more than two years to deal with the mess left by others over a half-century."

The comment, in the opening passages of Reagan's annual speech to the Conservative Political Action Conference, was one of the president's strongest signals that he is at least contemplating a second term. Some conservatives regarded it as the code words they have been wanting Reagan to utter for the last several weeks.

But Reagan did not take the extra step of saying that it would take six years to carry out what he called "the unfinished agenda" of conservatives. And the fact that he referred to "our cleanup crew," rather than to himself, left open a question that the president repeatedly has said remains undecided in his own mind.

Reagan blazed little new ground in his speech last night, but he did declare that his support of tax indexing was "non-negotiable," and he made a strong statement on the 1981 attempted assassination of the pope.

"The attack on his Holiness, Pope John Paul II--an act of unspeakable evil, an assault on man and God--was an international outrage and merits the fullest possible investigation," Reagan said. "Tonight, I want to take this opportunity to applaud the courage and resourcefulness of the government of Italy in bringing this matter to the attention of the world."

The remark brought one of the loudest bursts of applause during the speech, and the president then inserted a sentence not in his prepared text: "Contrary to what some have suggested--you can depend on it--there is no one on our side that is acting embarrassed or feeling embarrassed because they're the Italians going ahead with that investigation."

Some published accounts have suggested that the Central Intelligence Agency tried to soft-pedal reports that the Soviet KGB, when it was headed by the present Soviet leader, Yuri Andropov, had been involved in the attempt on the pope's life.

Reagan's speech last night was described by one of his political operatives as "a tightrope-walking operation" in which he sought to demonstrate that he has not abandoned the conservative faith while avoiding any rhetoric that would undermine his current attempts to form a "bipartisan consensus" in Congress.

Gently reminding his audience that conservatives are in power, Reagan said that it is "now our task as conservatives not just to point out the mistakes made over all those decades of liberal government--not just to form an able opposition--but to govern, to lead a nation."

Earlier, Reagan spoke more bluntly on the same theme in an interview with the weekly publication Human Events. Asked whether he was bothered by conservative attacks on him, the president said:

"Now, we're not such a great majority in the world, we conservatives, that we can be giving ourselves political saliva tests all the time. And I guess what bothers me the most in the whole thing is that some of the voices that were raised on the right against me weren't truly devoted friends to begin with."

In much of his speech last night the president relied on themes he has used before conservatives audiences in the past. He attacked "big government" and called for tough measures against criminals. And he supported other unachieved items on the conservative agenda: school prayer, anti-abortion legislation and tuition tax credits.

He also described the Soviets in militant terms, though less belligerently than he has in the past to the same audience.

"We have focused world attention on forced labor on the Soviet pipeline and Soviet repression in Poland and all the other nations that make up what is called the 'fourth world'--those living under totalitarian rule who long for freedom," Reagan said. "We have publicized the evidence of chemical warfare and other atrocities in Cambodia . . . and in Afghanistan. We have pointed out that totalitarian powers hold a radically different view of morality and human dignity than we do."

The president was applauded when he catalogued Democratic accusations against his economic program and then proudly took credit for reducing inflation and promoting economic recovery.

"It's time to admit our guilt, time we admitted that our liberal critics have been right all the time and they should go right on telling the American people that the state of the economy is precisely the fault of that wicked creature, Kemp-Roth, and its havoc-wreaking twin, Reaganomics," the president said.

But he misread the word "twin" in his text, instead calling it "truth."