They were outsiders who finally became insiders, but many of the conservative crusaders who came into office with President Reagan have grown frustrated and are resigning in increasing numbers.

"The crusade is over," said one administration official who has returned to private business.

Norman B. Ture, one of the leading supply-siders at the Treasury Department, departed last week for the Heritage Foundation, the conservative think tank. Another supply-sider, Paul Craig Roberts, has left Treasury for the Georgetown University Center for Strategic and International Studies.

Martin Anderson left the White House months ago to return to the Hoover Institution. Others, less well known, are finding the constraints and compromises of government work not worth the trouble; still others are staying on despite frustrations.

They came to government with hopes that the Reagan years would be a chance to get their conservative, free-market, supply-side, anti-Soviet convictions translated into national policy. Coming largely from academia and think tanks, where they'd been on the outside for years, they found that being on the inside was both exhilarating and excruciating.

"This administation has many, many more of these kinds of people," said Willa Johnson, a senior vice president of the Heritage Foundation who spent six months in the White House personnel office. "They are convinced their ideas will work, but they're not used to thinking in the political terms that an administration has to look at. They become impatient."

One of those recently caught up in the collision between conservative theory and practical politics is E.S. (Steve) Savas, a Columbia University professor who has built his career around the idea that private enterprise should rescue America's troubled cities.

When he came to the Reagan administration as assistant secretary of Housing and Urban Development, Savas helped write this theme into the draft of the administration's first statement on national urban policy. The document concluded that cities had been weakened by federal aid and their future lay in "greater reliance on the free market."

But when the draft policy statement became public, touching off a furor among the nation's big-city mayors, Savas was suddenly out in the cold.

White House officials announced that the policy had been sent back to the drawing board for "more research." Secretary of Housing and Urban Development Samuel R. Pierce Jr. promised the mayors it was only a draft and would "remain a draft."

The episode offered a glimpse into the ideological conflict that simmers between the administration's more pragmatic side and the conservatives who came to government determined to carry out a revolution from within.

This struggle for the administration's soul predates Inauguration Day, but lately some conservatives have decided to carry on the fight as outsiders.

Typical of them is Steve Hanke, a professor of applied economics at Johns Hopkins University in Baltimore, who took a leave of absence in June, 1981, to serve as a senior staff economist at the president's Council of Economic Advisers.

"What attracted me was that the president had an excellent blueprint," said Hanke, who was enthusiastic at first. "Those first few months it was a hard charge--and everyone was on the same wavelength."

But the euphoria turned to doubt. Hanke said he felt that the administration didn't carry its economic revolution far enough in cutting taxes and government spending. He said he was discouraged when his "free enterprise" ideas for helping the timber industry with federal timber sales didn't get off the ground.

"There's a large gap between the rhetoric and the reality," he said. "Now, we're back to business as usual."

Hanke resigned and returned to Hopkins with the feeling that "there are a lot of Republicans in Washington, but not a lot of Reaganites."

Joe Churba puts it another way: "Reagan without Reaganism." Churba wrote background material on the Middle East for Reagan the presidential candidate in 1980, and later went to the Arms Control and Disarmament Agency. Churba takes a hard-line view of the Soviets and said he went into the administration with the conviction that "arms control in itself" should "not be the centerpiece of Soviet-American relations."

But he became "gradually disillusioned," he said, because, for one thing, "Arms control became the administration's panic reaction to the nuclear freeze movement."

Churba resigned May 7 to return to the Center for National Security, a hawkish think tank here. He has no regrets about leaving the government: "Outside is the only place to be."

When Paul Craig Roberts and others began popularizing the idea of supply-side tax cuts in 1975, "we were a minority within a minority," he recalled.

As assistant treasury secretary for economic policy, Roberts was a principal architect of the supply-side tax cuts Reagan won from Congress last year.

Roberts said he expected that the "revolutionary change" he wanted would have to be won very early--or not at all. "You have to get your change up front, right away, before the PR people take over," he said. "And that's what we did."

But he grew angry and frustrated when many advisers urged the president later to raise new taxes. "I think the policy is in the process of being unraveled primarily by Republicans," he said.

Ture, the other supply-sider at Treasury who was outspoken against new taxes, has said he is leaving government because he needs to earn more money.

Martin Anderson, who was Reagan's domestic affairs adviser before leaving the White House to write a book, said the attraction of being an insider dimmed after a year.

"In any new administration 80 to 90 percent of what you will accomplish comes in the first year, and from then on you are making adjustments," he said. "There also comes a point where you have to decide if you want to be a public bureaucrat or an academician. I had one of the best government jobs--but it was still a government job."

Just as the conservatives are drifting away from the Reagan administration, so were many liberals and consumer activists dissatisfied when Jimmy Carter was in the White House.

For instance, Joan Claybrook had a celebrated falling-out with her friend and fellow consumer lobbyist, Ralph Nader, when she was Carter's director of auto safety.

The activists "come into a powerful post and think they just issue an edict and it happens," said Claybrook, who has settled her differences with Nader and is now president of Public Citizen, Nader's umbrella organization. To survive the trench warfare inside the government, she added, "You have to be a very good politician."

The administration's clash between ideology and politics was thrown into sharp relief by the urban policy flap.

The urban policy document was drafted under the direction of Savas, who believes that cities would do well to let private enterprise take over some of their functions and that the federal government ought to take a minimal role in rescuing ailing cities.

Such thinking remains anathema to most of the nation's big-city mayors. Detroit's Coleman Young, president of the U.S. Conference of Mayors, describes it as a "disposable cities" policy.

Those who worked under Savas said he insisted that the document endorse minimal federal help for the cities, despite objections from some White House aides who saw it as a political time bomb that could play into Democratic criticism that Reagan has turned his back on the poor.

Savas "had a free rein in this thing. His views were the overriding views, and anyone else's were treated marginally," said a HUD official who worked on the draft policy statement.

Robert B. Carleson, special assistant to the president for policy development, also worked closely with Savas on the policy statement, the HUD official said.

Savas says: "I didn't come here . . . with a hidden agenda or a personal agenda. I came here to do what Reagan wants."

Among the first draft's conclusions were that federal aid had weakened cities and should be curtailed. It also said the nation's mayors had become "wily stalkers of federal aid" and that cities should not provide services such as day care because it "has traditionally been performed well on a voluntary basis by relatives or friends."

Savas refused to discuss these conclusions last week except to say he later made revisions he would not specify.

The thrust of the document was preserved in a summary that made it to a Cabinet Council on Human Resources meeting June 18 that Reagan attended. Savas was in Paris, and the White House officials who were worried about the political fallout used the occasion to suggest that urban policy needed more work and should be sent back to the drawing board.

Without examining the summary closely, Reagan agreed, according to Rich Williamson, assistant to the president for intergovernmental affairs.

Published accounts of the document appeared a few days later, leading to a broadside from the mayors. The White House response was to try to minimize the political damage and disavow the draft policy.

"The irony," said one HUD official, "is that there is nothing in that policy that does not reflect administration philosophy."