Despite President Reagan's earlier instinctive mistrust of arms control agreements, he is demonstrating new flexibility in his approach to the strategic arms reduction talks (START), which reopened Wednesday in Geneva.
While Mr. Reagan's new proposals may be insufficient to bring the Soviet Union to an agreement, they are winning him points in Western Europe and in the United States Congress, and that was surely one of his aims. A large liberal group of Democratic congressmen still requires some convincing.
But arms control specialists in Congress say the President is on his way toward gaining the bipartisan consensus on arms control and defense that he has so often advocated recently.
Many of the liberal and moderate Democrats, whom Reagan needs if he is to build that consensus, say they still don't completely trust what some see as the new Reagan. But a number of them say the trends are in the right direction and they want to encourage them. They say they realize that the prospect of the 1984 presidential election, as well as the need to get support for the proposed MX missile, may have helped drive Reagan toward arms control. Polls tend to show that it would help Reagan's reelection prospects if he would show greater determination to get an arms control or arms reduction agreement. But if that's what it takes to get an agreement, so be it, some of these Democrats say.
In a three-page statement Wednesday, Reagan said he had directed the chief US negotiator in Geneva, retired Gen. Edward Rowny, to raise his earlier proposed limit of 850 deployed ballistic missiles. The aim would be to allow both sides to move toward the development of small, single-warhead missiles, dubbed Midgetmen in the US, and away from heavy, ''destabilizing'' missiles carrying more than one warhead.
Reagan didn't give enough details about proposed sublimits for heavy missiles to satisfy his Democratic critics. They say they plan to press the administration on this point.
The US proposal still appears to require the Soviets to make radical reductions in their heavy, land-based missile force, which is where they have made their biggest nuclear investment and where they hold the advantage over the Americans.
In dealing with the issue of throw weight, or the lifting power of missiles, Reagan handed a tentative victory to State Department and National Security Council staff pragmatists over Defense Department ''ideologues.'' The President said that the throw-weight issue would have to be addressed in START. But he also indicated that the problem could be dealt with through the indirect route of agreement on other aspects of his modified arms control package. The Soviets are said to hold a 3-to-1 advantage over the US in throw weight, or what the President described as ''destructive capability.''
The Defense Department, spearheaded by Assistant Defense Secretary Richard Perle, has consistently made throw-weight limits a high priority. Congressmen who were briefed Tuesday on the new Reagan proposals described Mr. Perle as ''hopping mad'' and ''pounding tables'' over the proposals.
US Rep. Albert Gore Jr. (D) of Tennessee, who within a few short years has turned himself into one of the top congressional arms control specialists, praised Reagan's modified START proposals.
A leading advocate of Midgetman and a key member of a group of some 20 Democratic congressmen who are leading the House move toward bipartisan consensus, Representative Gore says he believes the President has brought the nation closer to an arms control agreement with the Soviets by placing a new emphasis on ''stability'' in the nuclear balance and greater flexibility on throw weight.
Reagan's 1980 campaign rhetoric emphasized heavily the ''fatal mistake'' the Carter administration made in changing its arms control positions in the face of Soviet intransigence. But Reagan himself seemed to be doing just that - albeit under bipartisan cover.
Some specialists predict no progress in the START talks until there is progress in the negotiations with the Soviets on European-based intermediate-range missiles. But Mr. Gore argues against too pessimistic a view. He says that on more than one occasion, well-placed Soviet officials have shown an interest in shifting from multiple warheads toward a ''stabilizing'' balance of small, single-warhead missiles.
Some specialists agree that Reagan now has an enhanced chance of getting an arms control agreement, thanks partly to the unified stance taken by the NATO allies at the recent Williamsburg, Va., summit and the expected reelection victory of British Prime Minister Margaret Thatcher. Her victory would give key West European nations an almost undivided phalanx of conservative governments.