President Reagan's nomination of Kenneth Adelman as his top arms control official hangs by a thread over the basic political fault line in the Republican Party - the split between conservatives and moderates.
Mr. Adelman himself, despite detractors' complaints he lacks experience and serious commitment to arms agreements, is more a pawn than the cause of the threatened nomination holdup.
And the affair reaches beyond domestic politics. A narrow win or loss for Adelman in the Senate Foreign Relations Committee - even if Adelman were to win confirmation on the Senate floor later - would signal Europeans that Mr. Reagan's base of support in Congress for an arms agreement could prove soft. The spectacle of Adelman's nomination dangling in the air while West Germans prepare for their March 6 elections is unsettling. Placement of US missiles in Europe figures prominently in the German race.
Why Reagan would have gotten this far out on a limb with the Adelman nomination puzzles many Washington observers.
The risk, domestically, is to further rile GOP moderates in the Senate and House. They've already become more independent about environmental, economic, and social policy disputes surfacing in the administration. Moderates are watching the developing inquiry into the Environmental Protection Agency's toxic waste actions. The White House decision to appeal a federal court ruling against the so-called ''squeal rule,'' requiring notification of parents when teens seek birth-control devices, also concerns them.
''Reagan holds the cards'' with his right wing, one GOP adviser says. The far right has no choice but to back Reagan if he runs again. Indeed, criticism from Reagan's right makes him look more moderate.
The Reagan record on standing by appointees, moderates or conservatives, is quite clear. Though targeted by conservatives, moderates on his staff, like chief of staff James Baker III, continue to thrive.
Reagan stuck by his nomination of Richard Burt as assistant secretary of state for European affairs, a choice opposed by the right, and won Senate confirmation of Mr. Burt.
But the President will continue to say and do many of the things the right wants. He is addressing the Conservative Political Action Conference in Washington this week. Recent days have seen strong Reagan pitches for school prayer and tuition tax credits, as well as presidential statements excoriating abortion.
Democrats in Senate and House were largely dealt out of basic policy formulation early in Reagan's term. They're now working their way back into the picture on jobs, social security, defense, and other issues. But it's his own party that's giving Reagan the most trouble. Earlier, the right held up Burt's nomination. Now Adelman's is held hostage by moderate Republicans.
''The President has made it a personal surrender thing,'' says one Democratic official of Reagan's threat to take the Adelman nomination to the Senate floor over the Senate Foreign Relations Committee's objections. ''That might just be setting the stage for Adelman taking himself out. It's a pretty bipartisan concern about Adelman's qualifications - whether he's up to the job, and whether his nomination suggests this administration is serious about arms control.''
Neither side is able to predict the course of an Adelman vote on the Senate floor. ''Reagan has a Republican majority,'' concedes one Democratic strategist. ''He has enough chips he could call in. Maybe he could pull it through. But these guys are pretty independent in the Senate.''
Reagan would have to win over one of two Republicans, Senator Mathias and Sen. Larry Pressler of South Dakota, to gain a narrow 9-to-8 win in the committee. Mathias is seen as the harder to move, Pressler the potential ''weak link.'' The committee voted to put off its decision on Adelman to next week. This will give the White House time to work on Pressler, who is up for reelection in South Dakota in 1984, and do its spade work for a Senate floor fight.
''The easy way out - Adelman's withdrawal - is easy to see,'' says Charles F. Doran of the Johns Hopkins University School of Advanced International Studies.
''I don't think the Soviets are going to overread the Adelman events,'' Doran says. ''They see a President very committed to the positions he held before. They don't see a President weakening. Arms control for Reagan is but part of a larger military posture toward the Soviets.