Washington — Some long-time Reagan watchers say that the best way to understand the President is to refer to his years as head of the Screen Actors Guild. Joe Laitin, who may be the best government press secretary ever fielded in Washington, knew Reagan back in those years before he leaped into the race for governor. He says that from his observation post as a broadcaster and free-lance magazine writer, he was ''very impressed'' with the way Reagan handled his union duties. ''Reagan,'' says Laitin, ''had a good sense of tactics and timing.''
A Reagan associate of long standing contends that, while Reagan's commitment to conservatism and anticommunism is very genuine, the President is more of a strategist than an ideologue.
''He'll never go down to defeat on an issue,'' this informant says, ''if he believes that he can still win most of what he is after through accommodation.''
Indeed, new wisdom about Reagan is surfacing among some observers here who find in Reagan today the same skilled negotiator he became back in his union days. And they reach this conclusion from the way he has handled several big issues of late.
''He takes a tough position, one calculated to please the conservatives, and then moves to a conciliatory position,'' comments one of these Reagan interpreters. ''He's a master of knowing how to go for the possible.''
Reagan is pictured as believing this kind of negotiating accomplishes two objectives:
One, it prevents his hard-core conservative supporters from charging him, as he moves toward compromise, with being soft on communism or communists. And, two , it enables him to get the most out of the bargaining with the Soviets.
Reagan the ''negotiator'' is also being seen by some in his recent veto of Congress's budget legislation and his layoffs of government workers. From that tough position he now is portrayed as moving in a more conciliatory direction but for this reason: he believes he will get much more in the way of budget cuts out of Congress by first flexing his muscles a bit.
The canny union leader, seeking to get the most out of a controversy, also comes through in Mr. Reagan's firing of the air controllers. First, according to this perception, he nailed down the conservatives who were outraged by the controllers breaking their pledge not to strike as well as their violation of the no-strike law. And now he is providing some concessions to the controllers in order to win back some measure of friendliness from the big union leaders and union membership in general.
Of course, Reagan had held an antipathy for organized labor's chieftains long before entering politics. Bill Boyarsky, in his excellent book ''Ronald Reagan, His Life and Rise to the Presidency,'' says that through the years ''Reagan's loyalty to the Screen Actors Guild never wavered but he became more and more hostile to organized labor's national leadership.''
Also, Reagan has consistently expressed his strong opposition to communism and the Soviet Union, from the time of the early postwar period. So it was as Reagan the well-known ''hawk'' that he entered the arena with the Russian leaders. But those close to the President contend that his goal is and has been not confrontation but peace - and that he is prepared to give ground, after first establishing his toughness.
Reagan doesn't always talk tough with a negotiating maneuver in mind, of course. That wasn't why he gave the impression to senior citizens that he was about to cut their present or anticipated social security benefits. He's talking softer now. Not because of any prearranged strategy, but because of the angry protest from older Americans his earlier words aroused.
And when Reagan ordered Haig and Allen to stop their squabbling, this wasn't a tactic. Nor was he equivocating when he ordered Stockman to desist from loose talk.
But it appears that on major issues requiring political strategy Mr. Reagan leans on his experience as a union leader.