"I cannot forecast to you the action of Russia," Winston Churchill said in a 1939 radio broadcast when the Soviet Union was allied with Nazi Germany. "It is a riddle wrapped in a mystery inside an enigma."

Hard-core conservatives must be tempted these days to make a similar assessment of their champion, Ronald Reagan.

Instead of veering to the right as many expected him to do in a second term, Reagan has resolutely set about capturing the center ground in domestic and foreign affairs.

Last month, he proposed raising corporate taxes and lowering individual tax rates.

Last week, in an iconoclastic decision that he said walked "the extra mile" for arms control, the president surprised almost everyone by deciding to continue complying fully with the unratified SALT II treaty, which he had invariably described in campaign speeches as "fatally flawed."

Reagan followed up this action by promoting career Foreign Service officers favored by Secretary of State George P. Shultz rather than political appointees backed by conservative senators, by disavowing any intention to overthrow Nicaragua's leftist government and by deferring plans to sell Jordan as much as $750 million in arms over five years.

The president took these actions against vocal opposition from trusted conservatives.

Defense Secretary Caspar W. Weinberger, Attorney General Edwin Meese III and White House communications director Patrick J. Buchanan argued for a stronger response to purported Soviet violations of SALT II, and Weinberger also made a passionate plea for sending arms to Jordan.

When conservative senators visited Reagan in the Oval Office last Wednesday to voice complaints, Reagan listened quietly and said that, when he was governor of California, some Republicans preferred to "jump off the cliff with all flags flying" rather than accept political reality.

Reagan is not a jumper. His behavior last week summarized the contradictory nature of a public career in which he has been simultaneously a true believer who never tires of preaching the conservative gospel and a political realist who takes what he can get and comes back for more the next time. His strengths are that he is willing to trim and rarely gives up, a combination of qualities that has finally produced a limited victory for his policy of aiding Nicaraguan rebels.

Basically, Reagan has followed the same pattern since his first year as governor, when he accepted a huge and largely progressive tax increase after campaigning against increased taxes. In his second term, he described welfare fraud in apocalyptic terms and settled for a decent bill that raised grants to the poorest recipients.

Over the years, conservative and liberal ideologues have been able to maintain their stereotypes of Reagan by blaming or crediting his aides or wife Nancy for whatever he does.

It is tempting to do because Reagan is passive by nature and a delegator by inclination and necessity. But his behavior does not seem to depend on who heads the supporting cast.

In California, the role of pragmatist was often played by Meese or Weinberger, now heroes of the conservatives.

During Reagan's first term as president, his most sensible decisions were often attributed to chief of staff James A. Baker III.

Now Baker is at Treasury, and Reagan is proving equally realistic under the management of a White House team headed by Donald T. Regan.

Shultz, conservative enough for most tastes, has replaced Baker as the moderate bogeyman leading Reagan down the primrose path.

My view, after watching Reagan for two decades, is that he always finds people in his inner circle who will argue the pragmatic case and that he usually listens to them.

Debate often rages among his advisers, and, from that, Reagan frequently distills the sense of an issue and finds a middle ground.

Reagan is a conservative and will continue seeking ideological objectives throughout his presidency. But he inhabits the real world.

He remains a riddle and an enigma to many of his followers because he intuitively understands better than they do the difference between campaign rhetoric and objectives that are politically attainable.

Reaganism of the Week: Telling Indian Prime Minister Rajiv Gandhi during a photo session how he had said that taxes would be increased "over my dead body," Reagan motioned toward reporters and said, "I don't want them to take me seriously."